Looks like little ferns? Nice thick mat of green and blue that looks pretty in the light?
Its horrible.
Pull the rock out, clean the algae off in a different container (with water from the tank). DO NOT try to clean it in the tank. The small stuff that gets loose will only take off in another area, its gets worse from there.
If you let it grow, that whole rock will be nothing but algae in a couple weeks.
If this is all you have- try to chip or scrape away the surface of the rock under it. Bryposis (sp?) is a tough algae to kick once it has a foothold.