What's growing on my rock?

[NewestToTheReef]

What's the dark read growing on my rocks? Its only on the top sides of the rock. It wont blow of but will come off if I really scrub it with a toothbrush. Its only on the rocks.

 

[Brit’s Fish]

Coralline algae.

 

[Rmckoy]

If it’s hard to remove I will also say coralline .
I find it a little odd that it’s not on the glass or any black plastics in the tank .
It’s a good thing ! Congratulations