I dont think its unfair to the seahare to put him in a tank full of green hair algae. As far as I see it, if you had a problem as extreme as I had, its OK to put one in your tank as long as you dont have intentions of keeping it. I agree, it would be unfair and "inhumane" to keep one as a full-time inhabitant, but, if its there for the sole purpose of helping rid of algae and you intend to pass it on to another reefer or back to an LFS store, what is the harm ?
Ive had mine in my tank for about two weeks now, and my hair algae has gone from waving grass, to a few tufts on some rock. Granted, my tank was just redone, and Im going through a normal cycling period, but I dont think its wrong to try to clean it up with a creature born to do just that.
As long as you are doing regular water changes and are using RO water, I think its OK to add one temporarily.