It seems to be green hair algae.
emerald crabs have been known to eat hair algae but sometimes they cant keep up with it.
I would suggest making sure your nitrates are low and be sure to cut down on the nutrients/ supplements being added into your tank since a high nutrient level will help hair algae grow like crazy. Also, sometimes overfeeding and too many hours of light per day can cause that algae to bloom so just make sure to watch everything and see how it goes. Scrubbing the rock will help remove some of the algae (dont scrub it in the tank since it will spread the algae) but often times scrubbing is only a short term solution,
mike15.