Hair algae is brought around by high nutrients, nitrates and phosphates.
To get rid of Green Hair Algae you need to starve it. Basically by reducing nitrates and phosphates. Also you want to make sure that your lights aren't old, as low levels can cause Green Hair Algae to grow. Other ways it to pick it off the rock or if possible remove the rocks and scrub it off. This will help keep it under control but you still need to get your levels under control, otherwise it will comeback. Most fish wouldn't eat it, though some Rabbit fish and foxfaces will. Sea Hares will eat it but once it's all gone they will died of starvation, they also will release red ink into the tank if attacked or caught in a power head. Regardless of whether fish, sea hares or crabs eat it you still need to keep the nutrients in check, otherwise it will keep coming back.