Mustard algae, and why brushing it away is not the same as killing it

Telling it apart from green algae and from dirt

Mustard algae is frequently mistaken for sand or settled dirt, which is why people vacuum it and consider the matter closed. Three things distinguish it.

The tell is the last row. Something that returns to the same shaded patch within a few days of being cleaned is biological, not sediment. And the fact that the water can stay clear while it is happening is why it goes unnoticed — unlike the green bloom our algae guide covers, this one does not announce itself by ruining the water.

Why the normal routine does not clear it

Ordinary maintenance chlorine at the CDC’s published minimum — at least 1 ppm free chlorine, or at least 2 ppm where a stabilizer is in use — keeps ordinary algae from establishing. Mustard algae tolerates a good deal more than that, which is exactly why it survives in a pool whose readings look fine.

The practical consequence is that this cannot be treated at maintenance levels. It needs a deliberately heavy shock, and half measures genuinely achieve nothing — you spend the product, the algae survives, and it is back at the weekend.

There is a second reason people fail here, and it is cyanuric acid. If stabilizer has climbed through a season of trichlor tablets, your chlorine is sluggish even at a good reading — the exact scenario our cyanuric acid guide describes. Treating stubborn algae in a pool with high stabilizer is fighting with one hand tied; a partial drain first is frequently the fastest route.

The treatment, in order

1. 1

   Test first, including stabilizer

   Free chlorine, pH, total alkalinity and cyanuric acid. If stabilizer is high, replace a portion of the water before treating — about $8 of water for a third of a 3,955-gallon pool at the national average rate. Correct alkalinity, then pH, before you shock; chlorine works poorly outside the CDC’s published 7.2 to 7.8 pH range.
2. 2

   Brush every surface, thoroughly

   The whole wall, the water line, the floor, and specifically the shaded and low-circulation areas where it lives. Brushing is what exposes it to the chlorine; shocking an unbrushed pool treats the water and not the problem.
3. 3

   Shock heavily, at dusk, with the pump running

   Unstabilized calcium hypochlorite is the right tool here: it hits hard and does not add more stabilizer to a pool that may already have plenty. It adds calcium hardness instead, which matters little on a seasonal vinyl pool you drain each autumn. Dusk, because sunlight destroys unstabilized chlorine quickly.
4. 4

   Filter continuously, then change the cartridge

   Run the pump without a break until the water is clear. Then replace the cartridge rather than rinsing it — it is now full of what you just killed, and a rinsed cartridge is a reinfection route. Our cartridge guide covers the interval.
5. 5

   Brush and vacuum again the following day

   Dead algae settles. Vacuum it out rather than letting the filter deal with it, particularly on a small pool where a single vacuum session can load a cartridge on its own. Our manual tools page covers doing it slowly enough not to stir it back up.
6. 6

   Treat everything that went in the pool

   This is the step that decides whether it comes back. Floats, toys, the skimmer net, the vacuum head, the pole, the ladder steps, swimwear. Anything porous or textured holds it. Wash them and leave them in the sun to dry completely.
7. 7

   Then dose a preventive algaecide

   Polyquat, at the label rate, as prevention rather than cure. It is non-foaming unlike quat products and contains no metals unlike copper-based ones, which is why it is the sensible default on a vinyl liner.

Keeping it away

Fix the dead spot. If it always returns to the same place, circulation is poor there. Angling the return jet to sweep that area, or simply brushing it weekly as part of the routine, removes the habitat.

Brush weekly, including the shade. Our weekly schedule puts brushing before shocking for exactly this reason, and the water line and shaded wall are the parts people skip.

Watch stabilizer across the season. A pool whose chlorine is sluggish is a pool where this gets a foothold. Test cyanuric acid monthly and dilute when it climbs — our midsummer reset is the scheduled version of that.

Do not go looking for exotic causes. Phosphates get blamed for stubborn algae constantly, and in a pool with adequate, responsive chlorine they are not the constraint — our phosphates guide is the skeptical version.