For hair algae, I started by removing as much as I could using a toothbrush (I don't think you can expect the critters to remove it all if there is a lot) and then added a few peppermint shrimp, hermit crabs, and an algae-eating lawnmower blenny. It worked really well.
If your tank is big enough for a bristlenose or yellow tang, that would work well too.
I tried lettuce nudibranch, but they didn't seem to do too much and left a lot of slime tracks.
Sea hares may be the exception. One speciman alone can tear down a huge amount of hair algae, but then what do you do once there is too little to keep it alive?