Ohhhhh yes, the dreaded green hair algae. It is a pain and normally a product of excess nutrients in the water, most commonly PO4. The best way to get rid of it is by manual removal. Take the rock out of the water and srub it off and rinse it in SW before putting it back. Test for PO4, evaluate your water source, feeding technique and schedule. These all contribute. Take care of it now or it will become a much bigger problem.