Brown stain at water level during SLAM

[binovc]

Starting day 3 of SLAM.
Noticing a brown stain on the vinyl at the water level. The stain is only about 1/3 of the way around the pool. The stain is concentrated just past the return outlet. It does not brush off.
What would be causing this?

 

[Texas Splash]

I'm thinking one of two things:
1 - An organic scum line which can feel gummy and be a bit stubborn. Often times can be removed with a mild scrub pad and a water/bleach mixture or Magic Eraser.
2 - Iron staining. Even though you're in the city, it's not uncommon. The elevated FC level could've cause iron to precipitate out to the side. Try rubbing the area with some crushed vitamin c tablets in a thin sock or nylon to see if it changes.

 

[binovc]

Thanks, will check out the solutions.