Bit tough to see from the picture but is it fuzzy and pull off easily? If it's not, probably Gracilaria, otherwise I think it may be basically a nuisance algae. I saw it from time to time in my old macro algae tank when I was a bit late or lazy with maintenance. Only upside I ever saw to it was that for some reason pods LOVED living in it. You could actually see them crawling all over and in it.