I am hoping for some help in determining if this is black algae or something else?
I have been dealing with this issue for a few months now. These black stains began appearing around a few of my pool returns. It is not all, just 2-3. When you rub your hand over the black spots some of the black rubs away and you can see a little gray/black on you finger. It is little smooth/slimy at first touch but doesn't exactly a slick as normal algae.
I have used wire brush to get remain black off and it general disappears after a few passes. I have tried rubbing chlorine pucks over after wire brush and the black comes back within days. I have also shocked several times up to 35 PPM and have maintained the chlorine higher at about 12-13 PPM however this black keeps reforming if I don't brush in the those spots every day with a wire brush. The rest of the pool looks great.
Here is what I have:
Pool 1.5 years old
Chlorine:21 (Shocked last night)
Any ideas what this is? Is it black algae or something else? The pool guys say to use Copper Algaecide however based on my readings on this site that does not sound wise. Here is a picture of it. I appreciate your help.