Any idea what this is or how to get it off liner?

[Mattou01]

It’s only around where liner goes into groove. On 1 side of pool- East side. Side that gets most sun.

 

[TinFoilHat]

I would take the pool up to SLAM level and then using diluted liquid chlorine scrub the heck out of it with a nylon brush.

 

[Newdude]

Its mold/mildew. If the bleach isn't getting it, try vinegar, or mix up 1 tsp of baking soda in 2 cups water and give it a scrub. I'd probably use a toothbrush being such a small line of it.

 

[Mattou01]

Vinegar didn’t work.

 

[Newdude]

Ok then it's baking soda paste and toothbrush time. I'd skip right over the mostly water version myself, seeing how bleach or vinegar didn't work.

 

[Mattou01]

The baking soda paste didn’t do anything. I’m guessing it’s mildew but I don’t know