So the hair algae uses nutrients as it grows. Nutrients enter the tank from feedings usually. If all that's going on is the sea hare eating it, then you're not going to get anywhere, because the poop has most of the nutrients in it still and well...there goes the cycle.
Try 2 days of no light, with the sea hair hard at work, and on the first day rip out as much as you can (and take it out of the tank). You have a skimmer on that tank?
Also, lights don't have to be on much more than 7 hours (differs by opinion, but mine run 7 hrs actinics, and only 5 or so w/ daylights). And also, how's the flow in the tank?
So overall, be patient, cut back/stop feeding for 2 days or so w/ lights off and do as much manual removal as possible. After 2 days, do a water change and then lights on. For kicks, might as well test your water change water as well for nitrates/phosphates. They probably test low in the tank right now because the algae is using up so much of it. Oh, and check you bulbs too, old PC's have been known to contribute to algae growth.