Black dots on bottom area of linner

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 ???

Do the spots feel slimy? Can you scrape any of it up under a fingernail and smear it on a piece of white paper?

There is no difference to the fell of the spots or the normal liner. They wont wipe, scrap or in any way be diminished by scraping with a finger nail, or a rubbing with a fine nylon scouring pad. They look like a sharpie has been used to make them! At a few of the spots it looks like it my be degrading the integrity of the liner under the spot.