I think lawnmower blennies prefer the algal slime that grows over glass and such; at least mine doesn't seem to do anything about filamentous algae. But he sure keeps that pesky nori from ever getting out of hand.
I'm not sure what can do in a sea hare. Starvation probably. The first one I had lasted a long time, but he developed a taste for nori, so that probably helped. Eventually there was a run-in with a powerhead, and although it wasn't fatal, it seemed to cause enough distress that the worms did it in overnight. I have another one right now and it doesn't seem to touch the patches of bryopsis (or whatever patchy algae I seem to have in 2-3 spots that just won't go away), and doesn't seem to touch nori. I have no idea on which it is subsisting.