DON'T buy an fish/invert to fix it! Most likely it won't work! I learned the hard way... Sea Hare ($50), lasted 1week, still plenty of hair algae after he passed so don't know what happened. lawmower Benny/algae Benny ($25) wouldn't touch it! And now have a green emerald ($10) in my sump because he preferred to eat my corals instead of the hair algae! I agree with everything listed above, cut back lights & feeding. Do weekly water changes, maker sure to blow off your rocks too! And pick it out when you can. I've used old water and a toothbrush on some of the bad rocks (out of the tank!) and that's helped. Good luck be patient.
highly recommend finding the source.