Hmm that looks a little thick to be hair algae, possibly Bryopsis. I'm dealing with a hair algae issue right now and it's taking forever to get rid of it. Bryopsis is much harder to fix, and unfortunately I don't know enough about it to give you a solid answer how to fix it. There are several conversations on it, though.
Hopefully I'm wrong and it is hair algae, but from my experiences, hair algae is very thin stands and don't have and defining detail, whereas Bryopsis is thicker and has very small ridges on the strands