I have black dots appearing only on the bottom of the linner on my above ground pool. I first noticed 3 or 4 of these last year. I was very late removing my cover this year, about July 15th due to install problems with plumbing on a SWCG.
Now there are a couple dozen from 1/16" to 5/16" in size. They look like they could have been made with a sharpe or magic marker. I went into the pool with a fine nylon scouring pad and using my foot I tried to scrub away a couple with no luck. The spots did not appear to get any lighter or deminish in size or depth of color from the scrubbing.
Any thoughts ???