What is This!

[mocha 75]

They’re forming all over the glass. Difficult to scrub off. Coralline?

 

[mocha 75]

 

[EnterName]

If it's difficult to scrub off, it's probably some calcareous algae. I'm careful to call it "coralline" because there are algae species that grow very similarly but aren't part of the "Corallinales" order.

Hobbyists would certainly call it "coralline algae" though, and it doesn't look like anything harmful to me, even if actual red coralline algae looks better in my taste. There are a few gray/green corallinales, so the color unfortunately doesn't tell us much.