Black dots on bottom area of linner

[Heavyhook]

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 ???

[aylad]

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

[Heavyhook]

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.