What is this growing on my rock!?

[Cichlid Chris]

Does anyone know what this is? It's a dark brown/ red color. It won't wipe off. It Doesn't seem to be spreading. I'm assuming it's some sort of algae. I do weekly PWC and keep my aquarium healthy. Could this be harmful to my fish?

 

[SonicsDC25]

What are your water parameters and how long has the tank been up and running? I have purple/red algae on my rocks and my mbuna graze on it all day, but yours is a deeper purple than I've seen. I'd rule out diatoms since you can't wipe it off as well.