is not a algae, it's a bacteria. It's much "flimsier" than a algae so low flow allows it to congregate in a location, usually where there is die off and excess nutrients for feeding. It can be prevented by maintenance and keeping organic carbon sources under control. Most medications that kill it are basic antibacterial remedies that kills the bacteria but releases its contents back into the water where it will just start growing again. Over the 26 years of my reef, I've fought it multiple times when I got a bit behind on maintenance.
Siphon it out, increase flow and keep up on water exchanges and/or use something like GFO
to reduce phosphates.
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