Rust-colored spots around spa drain

[Doug Burnett]

These rust-colored stains have started appearing in our pool recently, usually around the spa drain. They do brush off easily enough, but curious as to what the experts here think is the cause. Thanks!

 

[reggiehammond]

If it poofs, it's not a metal stain.
Overnight Chlorine Loss Test to confirm it is algae. SLAM Process to eliminate.