Any idea on these stains?

[Lanzz]

Have never had stains like this before. They appeared a few weeks ago - it appeared to correlate with higher than normal PH and then adding acid to lower PH back to normal levels. Right now, FC is 3 and PH is 7.2 They don't scrub off with a brush. I did not think they were algae because they do not appear in the normal places that I would see it (we keep a white ball in the pool which is usually where algae appears first., and there is nothing on the ball.) Thoughts welcome...

 

[JamesW]

Probably the most common form of liner staining is that caused by bacteria or micro organisms growing behind the liner.

What are the black stains on the floor of my liner and how do I get rid of them? | McEwen Industries

If you have black or grey cloud-like stains on the floor of your in-ground vinyl liner, they are most likely caused by bacteria that is living in the ground under your pool. Bacteria sometimes show up years after a pool has been installed. We think the bacteria can be washed into your pool area by

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