Mustard algae: the yellow film that survives normal chlorine

How do you know it is mustard algae and not pollen or dirt?

Brush a patch and wait a day. Pollen disperses into the water and gets skimmed and filtered out and does not come back to the same place. Dirt clouds up and settles anywhere. Mustard algae brushes off in a sheet, the wall behind it looks clean and obviously different, and by the next afternoon the film is back in exactly the same spot.

Location is the second tell. Mustard algae wants shade and stillness. The north wall, the steps, under the diving board, behind the ladder rails, the low corner where the return current does not reach. Pollen follows the wind, not the shade.

Texture is the third. It looks like a fine dust or a pale yellow brown powder rather than a slick coating. Some owners describe it as looking like sand has settled on the wall, and they vacuum it away and find it back the next day, which is the most common way people discover they have it.

And critically, your chlorine has probably been fine the whole time. That is the part that separates mustard from green. A pool with a healthy free chlorine reading and a returning yellow film is a mustard algae pool.

Why does it survive chlorine that kills green algae?

Mustard algae, sometimes called yellow algae, is simply more tolerant. It will grow in water carrying a free chlorine level that green algae cannot survive in. Not immune, but tolerant enough that a normal maintenance target does nothing to it.

It also cheats. Mustard algae holds on in places your pool water is barely sanitized. Inside the filter, in the skimmer throat, on the underside of a ladder step, in the folds of a pool float that sat wet in the sun, on a wet towel, on swim trunks that never got washed. You can chemically clear the pool and reintroduce it from one unwashed inflatable the same weekend.

That is why treating mustard as a water problem alone fails so consistently. You have to treat the water and everything that touches the water in the same window, or you are just rotating the infestation between the pool and your gear.

What does an actual mustard algae treatment look like?

Raise free chlorine substantially above your normal target, well past the level you would use for a routine green shock, and hold it there for two to three days rather than one night. Set pH toward the low end of range first so the chlorine you added is in its more active form.

Brush every surface, every day, including all the places the film hides. Then brush the places you cannot see: pull the ladder if it comes out, brush the steps aggressively, and get the corners and the light niche.

Decontaminate the gear in the same window. Nets, brushes, poles, vacuum heads, floats, toys, goggles, swimsuits. Soak them in the pool while chlorine is elevated, or run the swimwear through a wash. This step gets skipped and it is the single most common reason mustard comes back.

Clean the filter at the end, and on a cartridge or DE system consider a soak rather than a rinse, because the media is where mustard survives a treatment. On a sand filter, a proper long backwash and rinse cycle. A sodium bromide based treatment is another route and it works, but understand you are converting your pool to bromine for a stretch, and that changes how the water behaves afterward. Decide that on purpose, not by accident.

Why does it come straight back?

The gear. It is almost always the gear. A pool cleared correctly and then re seeded from an unwashed net or a float that lived on the deck is the standard story.

The second reason is stopping too early. Mustard is tolerant, which means partial exposure selects for the survivors. A single night at an elevated level knocks the visible film down and leaves enough behind to repopulate within a week.

The third is circulation. If the same shady corner has weak flow, that corner is running at a lower effective sanitizer level than the middle of the pool, and mustard will keep choosing it. Aim a return eye at it, brush it weekly, and it stops being hospitable.

The fourth, on tablet fed pools, is stabilizer. Cyanuric acid that has crept high means your normal chlorine reading is doing less work than the number suggests, which gives a tolerant organism room to live. If mustard keeps coming back on a pool with good numbers, test stabilizer before you buy anything else.