Black algae or something else?

[Cronofl]

I recently purchased a home that has a 15 year old plaster pool. After moving in, I noticed there is a black/dark brown stain spot about 2 inch in length in the deeper end of the pool. Initially, I tried to scrub it with a metal wire brush. A tiny bit of comes off like faint brown dust and fades away in the water. I went to the pool supply store and the guy said it may be black algae and had recommended me with scrubbing the chlorine tablet on it. The chlorine tablet scrubbing didn't do anything to the spot. I felt my finger over the spot it and felt rough just like the surrounding plaster and not smooth or slippery. I wonder if it is some kind of rust or other organic stain.

[img]
https://flic.kr/p/2iEmezt
[/img]

[Phil.c]

Sounds like metal staining to me especially if the chlorine tablet did nothing but just to confirm, put a vitamin C tablet an it to see if that removes it.