Well you have a number of options:
If you want to go the "natural" route, scrub out the algae with a tooth brush. For a 28 gallon, Add 10 blue-legged hermit crabs, and 4 astrea, 4 trochus, and 2 turbos. The idea is if you have enough of different algae predators you may encounter the snail or hermit that likes eating that algae.
If you want the quick fix, use Algae FX and follow the instructions on the bottle.
I had a nasty hair algae problem that didnt go away for 4 months. Attacked it with everything I had (toothbrush scrubbing, GFO, heavy water changes, blackouts, extra clean up crew, sea hare,) when all of that didn't work, I used Algae FX. Killed off the hair algae. It grew back much slower the following week but I could see my clean up crew eating the new growth. Now I have no hair algae.