Tried a quick search but this topic is so varied. I have rust looking stains appearing at the bottom of my plastered pool. Not on the walls, just the bottom in random places. I put some vitamin C tablets on one of the stains and it actually turned darker! I jumped in my cold pool and attempted to wire brush it to no avail. I tried pouring bleach on a rag and holding it on the stain. Nothing.
I should add that I had something similar happen to me last year around this time. I didn't do anything and they ended up disappearing. But this year, there are many more stains.
What now? These are really ugly.
My FC is 6 ppm, pH was at 8, I've added some acid to bring back down and haven't retested. Alkalinity is 70 ppm.