I can attest that it will kill many corals in your tank.

I ended up with a bad frag once and within a week my tank was full of hair algae. I cannot, a year later, get rid of it at all. I've tried most everything. Removing rock and drying it, it lives everywhere unseen and quickly repopulates the rock. I've lost several corals, in fact almost all of my SPS due to it encroaching upon and right over living corals. I've tried rabbit fishes, foxfaces, tangs, snails, hermits, sea hares, overskimming, no lights, phos removers (exchanged media every other day for weeks), daily 5g water changes, pretty much anything I can think of.

The algae grows right up next to most of the corals. It then begins irritating the corals preventing them from extending polyps. In turn this limits their intake of food and light causing rapid degradation. Several LPS and SPS have died as the algae attaches and grows from the external skeleton. Over time it seems to almost "pop off" the LPS heads as they shrink and explore other areas to attempt a better living. I've heard of the algae leeching into the skeleton and using up the photosynthetic properties of the light that soaks through the flesh. This causes RTN and leaves behind a green skeleton instead of white.

Do I consider it bad? Beyond words. Nothing of my current tank will be put into use/display in my new tank.

The only control I have is to do weekly mass extractions. I shut off all flow and very carefully denude every inch of rock work of any and all algae blooms I can get. This can clean the rock for about 2-3 weeks if not kept up, but alas it does not seem to ever go away.