Black algae is what caused me to find troublefreepool and BBB about 10 months ago.
Since starting BBB I have not seen a trace of algae and my water is always crystal clear. I have carefully maintained my chlorine level.
Yesterday, I noticed a triangular shaped, black stain on a side surface. It is sitting right on a hairline crack. ( the pool is almost 20 years old).
It's about 1" long and 1/2" wide. It looked exactly like black algae, but it didn't feel like it. Black algae feels waxy. This didn't. It felt just like the plaster.
I tried rubbing it with a chlorine tab, then a vitamin C tablet and they didn't phase it. I also brushed it hard with a steel brush- no help.
I'm assuming it's some kind of mineral stain. I'm not going to worry about it unless it starts growing. It's 1' below the water line and if worse comes to worse I can drain down to it and scrub it with acid.
Any ideas about what it is?