It looks like Gelidium sp.which is a nuisance Red Hair Algae, they are wiry and problematic. Take it out if not eaten by a tang.
It also looks like Gracilaria sp. which is some of the finest macroalgae for marine aquarium use. Helps a lot with nutrient export. Gracilaria is an absolutely heavenly food for tangs. Do you have a tang to test it on?
Could be something else - what does the gurus say?
Got a small Regal Tang today. Will post pics later. My tank wont be big enough for it when its bigger but got a 700 x 700 cube in the pipeline. Thanks for the advice. (Hope my sixline leaves him alone)