Is this mustard algae?

[crashxx]

We have this in our plaster pool. What is it and what is the most effective way to kill it off? Thank you.

 

[Mdragger88]

Does it brush away? It may be metal stains. Here’s how to check different types of stains (metal or organic)

Stains in Your Pool

The stained pool is beyond embarrassing. Our in-depth Guide to remove stains in your pool will help you to remove the stains and prevent them in the future.

www.troublefreepool.com

Have u done an Overnight Chlorine Loss Test to rule out algae? If not I would do one tonight.

 

[crashxx]

Thank you. I will try that and report back.