Is this mustard algae?

[hemhaw]

I assume it's mustard algae, but it doesn't come off when I scrub at it. The vacuum gets some of it up, but not often. Chlorine levels have been stable around 6 for the summer. It is covered by a solar cover most of the time, and it struggles with a low pH, seems to stay around 7.4 due to this. CYA is around 50, and I recently added more because it's been pretty rainy here and I've had to waste out quite a bit of water this summer.

 

[Bperry]

I assume it's mustard algae, but it doesn't come off when I scrub at it. The vacuum gets some of it up, but not often. Chlorine levels have been stable around 6 for the summer. It is covered by a solar cover most of the time, and it struggles with a low pH, seems to stay around 7.4 due to this. CYA is around 50, and I recently added more because it's been pretty rainy here and I've had to waste out quite a bit of water this summer. View attachment 525327View attachment 525328

Looks more like staining. Have you done an OCLT to see if it’s algae?

 

[hemhaw]

I have not recently.
It's been there a few weeks and it's not really changed, my first thought was staining, so I was casually looking for ways to clean it off. It doesn't feel slippery like I would expect algae to feel, and all the levels seemed fine so I guess I just wasn't too concerned... It honestly just occurred to me today that it may be algae, so I will do that today

 

[JamesW]

Looks like iron.

Drop a vitamin C tablet on a spot.

How old is the liner?

 

[hemhaw]

I'll try the vitamin c drop also. Just have to get some.
I have no idea, we've owned it for 2 years. I don't think it's recent, though,

 

[JamesW]

The liner looks super old.

You should plan to replace it at some point in the near future.

 

[hemhaw]

So it appears to be staining, the overnight test dropped a negligible amount.
Yeah, I have the intention of doing it soon. I wanted to get the hang of pool care before replacement so I didn't have to worry much about bleaching it while learning.