Please help identify what type of stain this is

[Jlzan1]

Vinyl liner, algaecide and shock did nothing, continues to spread. Tried scrubbing it thoroughly, nothing changed. It started early July. Any help or suggestions would be appreciated.

https://photos.app.goo.gl/2iVRq56ecUe81Bmr6

[Phil.c]

Put a piece of chlorine tab on the stain to see if it's organic like algae, and a vitamin C tab to see if it's metal.

[CCJ12101]

In my pool, which is a diamond Brite plaster pool with chlorine, not salt, that stain would be cupric oxide. It’s caused by too much copper reacting with sodium hypochlorite (sanitizer or bleach). The copper is to kill the algae. I was able to get rid of it once with aluminum sulfate. The aluminum sulfate knocked my chemistry off for weeks, so be aware. Be able to use those metal chelating packs instead, and overtime pull out some of the metal. CuLator. I would consider all of the chemicals you have been using in your pool and go from there.