Looks either like dinoflagellates or cyano. The colour is closer to stereotypical cyano, but cyano can come in all sorts of nasty colours (black, purple, green). Dinoflagellates are usually brown, but I'm pretty sure that's not a rule that's written in stone either. The streaming filamentous bits makes me think dino, though. Is the mat of them on the rocks filled with air bubbles?
In either case they can be hard to tell apart. Dinoflagellates are significantly more difficult to get rid of, but here's a decent article on how if that's what they are:
http://www.advancedaquarist.com/blog...sons-i-learned
However, in a 5 month old tank I think you should expect this sort of thing to happen, so if I were you I wouldn't embark on a drastic or time consuming intervention just yet. For now I would just keep my parameters as good as possible, remove as much of it as possible manually, and let the tank keep figuring itself out.
Maybe someone else can give you a more conclusive answer as to species.