I've got every spreading grey spots on my plaster in ground pool. 13,000 gallons.
it started about 3 months ago wit the first spot I noticed, about the size of a dime. Now it's the size of a quarter and I have about 10-15 more.
I felt it, it have no different feel from the rest it the pool. It just feels like plaster. I've tried pumice stick, wire brush, and even used my power washer on it. Shocked (up to 12). Didn't even touch it.
The folks at the local pool stores, tried 3 different one, had no idea.
Metals, fertilizer, black mold. But no real idea. Do YOU have an idea?