First up, great forum and site! I've learned so much just from reading. But now I have an issue of my own and I don't know if it's algae or some type of corrosion. So I finally decided to break out my gopro and take some photos. I hope they're clear enough.
So, I've tried slamming the pool several times, always keep my chemistry balanced, even CYA in the 50 to 80 range (not great but I'm in the middle east so the sun is pretty intense). No matter what I do, even vigorous scrubbing by hand, it does not go away. The pool I think is fiberglass with a gelcoat finish, does this make sense? To make matters worse, I believe now it may have been painted as well with some heavy duty epoxy marine paint.
Please have a look at the photos and let me know. I'm thinking now to just drain it and pressure wash the whole thing and start from scratch.