I had a tuxedo urchin that cleared up some hair algae I had growing on the underside of my centerbrace (eventually I took some teeth out of the overflow to reduce the water level). He was a great addition to the tank, ate various types of algae, constantly plastered various rocks, caulerpa and stray zoos all over himself and (most importantly IMO) stayed small.
Currently I have a diadema urchin which arrived as a hitchhiker on a rock. He was about the size of a pencil eraser when I moved at the beginning of April. "So cute, I think I'll keep him" His test is now the size of a small mandarin orange and his spines are easily 12-15 cm long. So suffice it to say they are speedy growers. He also eats rock. He makes lovely little piles of granular sand all over the place. I often wonder how long it would take him to whittle all the rock in the tank down to just sand (not to mention how long it will be before I stick myself with one of his spines)
IMO, hair algae is one of those things that come on fairly quickly, and like everything else in this hobby takes forever to get rid of (ie. browning out of corals in like 2 days then months to color up again :confused). Between RODI usage and a phosban reactor (or even phosban in a filter bag in a high flow area) it should clear up on its own albeit slowly. It may help to pull some of the longer bits out by hand and then siphon any stray bits.