Hair algae problems

Is it only on one rock and can the rock be removed from the tank without disrupting the whole aquascape? If so you can remove the rock and clean it with hydrogen peroxide which will kill all the algae on the rock. Replace it and it should be fine. If the rock has corals on it you can spot treat the rock with a pipette or syringe. Some corals are fine with the treatment. I have done it on Zoo rocks with no problems. The zoos foam and retract for about a day then pop back open better than ever.

You can also increase your clean up crew and run carbon and phosban to remove nutrients form the tank.

I had problems like this in one of my tanks for almost a year. What finally did the trick for me was turning off all of the lights, putting a blanket over the tank for two days. This made the algae very week and I was able to pick it off with my fingers, and siphon it out. If your problem is just one rock though I would get the rock out before it becomes a headache. Just clean it off as they have said.
Good Luck!

emerald crab, lawnmower blenny, and i believe some tangs will eat it. Pull the hair out (not your own) and remember not to RUB IT IN! You need to pull it out and let the critters do the same. If it is on one or two rocks do as they say and pull the rock out. Check levels etc

If it's just normal hair algae the snails, crabs and herbivorous fish will eat it. if it is bryopsis then the only solution is to raise your magnesium levels to 1500+ ppm with Kent Tech M. You can tell it's bryopsis if it has fern or feather like stems. If it just looks like a wavy mass of green then it's just hair algae and your clean up crew should take care of it as long as you keep the lights and feeding down.