Hmm....looks like turf algae. Is it hard to remove from the rock? Some people like turf algae, some don't.....it depends on how much ya got. I had a bunch, added a chevron tang, and he tore into it. I think most of the bristletooth tangs (chevrons, koles, etc) will help take care of this while they scour the algae for their food. If your tank is big enough, a kole tang will probably help.