Black smudge on pool walls

[Meaty]

Hi
I have a black “smudge” on the wall of my pool in a couple of spots. It reminds me of rubber/brake residue. It comes off when I wipe it with my hand but brushing doesn’t seem to get it off. Any thoughts? Is it just dirt (lots of construction around here ) or something else?

My pool details
Fiberglass
Chlorine is 7 (usually have to add 1-2 ppm every 24 hours but that hasn’t changed)
CYA 40.

 

[Texas Splash]

Sounds like a scum line we can get at times. No real rhyme or reason. We can try to blame excessive oils in the water during swimming season, but sometimes it just happens. I usually get it once or twice a year on one corner and perhaps in the skimmer. We use a little elbow grease and perhaps a Magic Eraser to remove it on ours.

 

[Richard320]

Look closely at all of your pool tools.

When one of my brushes started getting worn to nubs, I started dragging the plastic spine against the plaster leaving blue stripes. It wasn't intentional.

 

[Meaty]

Thanks. That very well could be the problem as I’ve all but ruled out black algae