It almost looks like a combination of two. I'm no expert by any means. But it looks like some green hair algae, red slime maybe? And a strand of cyno. I know there are different types. I'm sure more experienced members will chime in with the correct I.D.
Here's a good read.
In any event I think most will agree that most algae is because of excessive nutrients.
Water changes will be needed. What are your parameters and light schedule?
I mentioned taking the rock out and scrubbing it only because I thought we were talking a small amount on one rock. This is clearly NOT the case.