What Is Mustard Algae and How Do I Treat It?

Mustard algae is a yellow to brown algae (sometimes called yellow algae) that settles on shaded walls, steps and floors, looks like a dusting of pollen or sand, and wipes away easily with a brush before drifting back down hours later. It survives normal chlorine levels that would kill green algae, which is why it keeps returning in pools that otherwise test fine.

How to tell it from dirt or stains

• It brushes off cleanly and clouds the water, unlike a metal stain
• It comes back in the same spot within 24 hours
• It favors the shady side of the pool and behind ladders and steps
• It is yellow, tan or brownish, not the blue-green of a normal bloom
• Water can look clear while the algae is present on the surfaces

If a spot brushes away and reappears in the same place tomorrow, you are dealing with mustard algae, not sediment.

The treatment that works

1. Balance first. Bring pH to 7.2 to 7.4 and total alkalinity to 80 to 120 ppm. Check cyanuric acid, because your shock target depends on it.

2. Brush every surface hard. Mustard algae hides in shade and under sediment. Get the walls, floor, steps, light niche, behind the ladder and the skimmer throat.

3. Raise free chlorine and hold it. Target roughly 40 percent of your CYA level (about 12 ppm at CYA 30, 20 ppm at CYA 50) and hold it for a minimum of 24 hours, retesting and re-dosing as needed. One gallon of 12.5 percent liquid chlorine raises 10,000 gallons about 12 ppm.

4. Decontaminate everything. This is the step people skip and the reason it comes back. Mustard algae survives on poles, brushes, floats, toys, robot cleaners, ladders and swimsuits. Soak removable items in the pool during shock treatment, or in a chlorine solution, and wash swimwear.

5. Run the filter 24/7 and clean or backwash it after treatment so you are not storing live algae in the media.

What if it keeps coming back

Persistent cases usually trace to one of three things: cyanuric acid over 80 ppm making your chlorine ineffective, a piece of gear reinfecting the pool, or dead circulation in a shaded corner. Some technicians use a sodium bromide treatment followed by chlorine for stubborn cases, which converts the bromide to hypobromous acid and is effective, but it does temporarily turn the pool into a bromine pool and should be handled by someone who knows the tradeoffs.

Copper-based algaecides also work on mustard algae, but they will stain plaster if the pool is not correctly balanced, so use them carefully.

Why the Keys see so much of it

Mustard algae likes warm, shaded, still water, and Key West supplies warmth every month of the year. There is no cold season to break the cycle, so a pool that never fully clears its mustard algae in June still has it in January. Our high UV also drives owners to run heavy stabilizer, and once cyanuric acid climbs past 80 ppm, chlorine loses the edge it needs against a tolerant algae like this one. Shaded pools tucked behind Old Town privacy fences and heavy landscaping are the ones we see it in most.

Treat the gear as seriously as you treat the water, or you will be doing this again in two weeks.