Blackish brownish around waterline by steps

[Schaeper]

We have been SLAMing the pool due to mustard algae. What is this blackish-brownish line around the waterline? It’s only near the steps and it wasn’t there this morning when I brushed the pool or at 11 when I checked the FC.

 

[ajw22]

Hard to see with the size pictures you posted - Picture Posting and Resizing - Further Reading

Looks like waterline scum to me. Try and clean it using a Magic Eraser on Pools - Further Reading

 

[Schaeper]

Hard to see with the size pictures you posted - Picture Posting and Resizing - Further Reading

Looks like waterline scum to me. Try and clean it using a Magic Eraser on Pools - Further Reading

I tried to do the original size and it wouldn’t let me so I just did small and it did that lol. Here’s a “large”. Hopefully it loads clearer. But it did come right off with a brushing. Just wondering why it randomly decided to pop up after a few hours