Brown stain on sun shelf

[Killfathobbit]

Just noticed it recently after hurricane Ian. Had no power for a week but pool never turned green. Any recommendations how to remove and what it could be from thanks

 

[reggiehammond]

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

You don't share pool math logs - how are you testing and how are you chlorinating?
It could be organic or it could be metal staining. Article linked above walks through the basic methods for ascertaining what it is...