I will try the vit c. test tomorrow.

I have no pool heater, and haven't used any copper algaecide. It did have a sandpaper grit consistency and seemed to appear after the deck was cut into and concrete dust was scattered.

(The dots seem clustered around the two areas that had cut concrete.)

Is there any kind of reaction concrete dust can have that would bond it to vinyl?

Is it possible that the mini-rebar supports / saw blade cast off could have oxidized and made the pattern? If so the vitamin C test should help identify.

Lastly there is still some very tough grit that is stuck on the liner. Would getting a 100% nylon scrubbing pad and using that on the liner be safe for the vinyl? The pool brush just isn't helping at all. I was only able to scrape it off with fingernails.