Mustard Algae: 9 Steps to Cure Your Pool



Algae is the bane of every swimming pool-owner’s existence, and mustard algae is one of the worst.

Once you have it, it’s especially difficult to eradicate, and you may have to deal with it over and over again. The good news is, there are ways to get rid of mustard algae for good.

What is Mustard Algae?

Formally known as “yellow-green algae” or even just “yellow algae”, mustard algae is known by its distinctive color and texture. Unlike green algae, which is slimy and clings to pool surfaces, mustard algae has a dry, powdery surface that is easily mistaken for dirt or stains on pool walls.

It also resists chlorine, and can survive outside of the water, so the tiny cells of algae can get carried from place to place on swimming pool equipment, water toys, and even on bathing suits. It’s no wonder that it is so hard to get rid of.

Where Does Mustard Algae Come From? 

Mustard algae is most common in warm climates, and lives in fresh water. The microscopic cells of algae often migrate into pools on the suits, hair, or water toys of people who have taken them into lakes and rivers.

It may also migrate from one pool to another, especially if pool equipment is used in different pools and isn’t sanitized in between.

What Does Mustard Algae do in a Pool? 

Mustard algae has a powdery surface that looks like dirt or even pollen, and is often mistaken for pool staining. In other words, it makes your pool look dirty and clouds the water.

Algae in a pool can not only affect your water quality to begin with, but will make it very hard to balance and maintain your water quality over time. Unbalanced water can affect the lifespan of pool filters and equipment, and cost a lot of time in testing and chemicals.

In the worst case scenario, a large algae infestation creates a welcoming environment for other bacteria and microorganisms, some of which, like E coli, can be harmful to a person’s health.

In other words, once you have this type of algae in your pool, you really do need to get rid of it.

How to Spot Mustard Algae

Mustard algae (or yellow algae) has a distinctive yellow color, ranging from bright yellow to a deeper mustard yellow. It looks dry and powdery, and is often mistaken for dirt or pollen.

This type of algae prefers the shade, and likes still water, so it’s more likely to be seen clinging to the walls or bottom of the pool. But even in pools with sunlight and good water circulation, the tiny cells of mustard algae can find and thrive in small “dead” spots, shaded areas beneath ladders and lights, and tiny crevices where they have the protection they like.

It’s also easy to dislodge from pool walls and equipment by brushing or scrubbing. In fact, it’s so easy to scrub away that many people think they have cleaned it away and solved their algae problem. However, mustard algae shelters in tiny, uneven surfaces in a pool wall, or behind the housings for lights, ladders, and other equipment, and will simply grow back after scrubbing. More intensive methods are needed.

How to Prevent Mustard Algae

Ok, lets talk about algae prevention.

The best way to get rid of this burden is to not have it to begin with. If you’re lucky enough to not have a mustard algae problem right now, I recommend following these steps to keep it that way.

  • Keep your pool balanced. This is always the first step in maintaining a healthy swimming pool. All algae thrive in high pH water. Keep your pH, alkalinity, and sanitizer levels healthy all the time.
Poolmaster 22272 5-Way Test Kit with DPD Tablets and Case - Premier...
Poolmaster 22272 5-Way Test Kit with DPD Tablets and Case - Premier...
    • Keep your water circulating. Run your pool pump and keep your filter running for longer periods of time. Most pools need an average of 8 hours of circulation a day to prevent algae, but larger pools need more time. If you live in a mustard algae-prone area, consider adding a circulation booster.
    • Keep your pool clean. Vacuum and brush clean your pool regularly. If you are in an area prone to mustard algae, you may want to invest in a robotic pool cleaner, because they are good at picking up fine silt and small debris and improve overall circulation.
    Milliard 14” Flexible Spa and Pool Vacuum Head, Great for Cleaning Debris...
    Milliard 14” Flexible Spa and Pool Vacuum Head, Great for Cleaning Debris...
      • Practice good sanitation habits. Use a bleach-based cleaner to regularly clean pool ladders, slides, covers, and other equipment. With suits, toys, and other swimming gear that may be used in natural water like lakes or ponds, clean them with a bleach-based cleaner and machine wash bathing suits before allowing them in your pool.

      In other words, you can prevent mustard algae by making your pool a difficult place for it to take hold and grow. Also, limiting algae growth will make it even easier to remove if the worst does happen.

      How to Get Rid of Mustard Algae

      If you have, or think you may have, mustard algae, here are three important things to remember before treating your pool water.You will also need to clean everything that may have come in contact with the algae, including pool toys, pool equipment, swimsuits, ladders, slides, and so forth. If you aren’t sure whether an item has been exposed to the algae, clean it just to be safe. 

      While bleach is an effective way to kill mustard algae on pool toys and accessories, pure bleach may damage some surfaces and plastics. Use bleach-based cleaning products or a diluted bleach solution instead.


        Mustard algae lives in the tiny irregularities, nooks, and crannies of surfaces. Whether you are brushing your pool, or using a bleach-based cleaner on pool accessories, it’s important to be careful and detailed in order to dislodge it.

        Step-by-step instructions to get rid of mustard algae: 

        1. Brush your pool. Brush the algae and the entire pool thoroughly, paying special attention to areas around fixtures, seals, gaskets, and beneath steps and ladders. Allow the algae to settle on the bottom.
        Swimming Pool Wall & Tile Brush,18' Heavy Duty Vinyl Polished Aluminum Back...
        Swimming Pool Wall & Tile Brush,18" Heavy Duty Vinyl Polished Aluminum Back...
          1. Vacuum your pool using waste mode.
          2. Balance your pool. Adjust it to a pH level of 7.2 – 7.6 and alkalinity of 80 – 120 ppm. 
          3. Brush your pool again. Dislodging any remaining algae will help to kill it when you shock your pool.
          4. Put all your hoses, maintenance, and cleaning equipment into the shallow end of the pool. That will help to sanitize it when you shock.
          5. Double-shock your pool. Add twice the amount of pool shock you normally would. You may also want to use a chlorine accelerator or a product specifically made for mustard algae. These additives use sodium bromide, which is more effective against this type of algae compared to pure chlorine. If you are using one of these products, follow the manufacturer’s instructions when shocking.
          DryTec 1-1901-24 Calcium Hypochlorite Pool Shock, 1-Pound, 24-Pack
          DryTec 1-1901-24 Calcium Hypochlorite Pool Shock, 1-Pound, 24-Pack
            1. Clean your pool filters. It’s not unheard of for algae to hide in your pool filter. Make sure to change “backwash” to “waste” when cleaning your filter.
            2. Balance your water and scrub your pool. For 3-5 days after shocking, maintain a high chlorine level, and continue to brush your pool.
            3. Shock it again. After a few days, shock your pool again at normal strength, just to make sure you have gotten all of the mustard algae. Again, if you are using a chlorine accelerator, follow the instructions.
            4. Use an algaecide regularly. Algaecide is a preventative, not a cure. It’s still surprising how few pool owners actually know this.

            That may seem like a lot of work, but mustard algae is so small and resilient that it is very difficult to completely eradicate it. Most people clean and shock their pool once, and think they have gotten rid of it, only to see it again a few days or weeks later.


            To get rid of mustard algae (or yellow algae), you need to attack it mercilessly on all fronts. Channel your inner John Wick and scrub your pool with a vengeance.

            Categories: Pool Care, Pool Problems