If your tank is over stocked, over feed, under skimmed, or maintained with tap water, you'll get hair algea. The only way to rid the tank of the algea is to remove the rock and scrub it clean (then rinse it in old tank water) and put it back in. Unless you have corrected the problem that caused the hair algea, it will come back right away.
Hair algea feeds off excess nutrients. A phospate test kit may show a zero reading, but is still present in the water, but the hair algea is consuming it and giving you the false impression because your test kit is not reading it.
Tell us everything about your set-up and maintanace routine, and we'll help you figure out what is causing your problem.
Louey