Most lawn mower blennies will not eat anything but hair algae. That is why many people don't recommend them. Bicolor blennies are supposed to eat algae and flakes so they can theoritically survive after the algae is under control.
Most emerald crabs don't eat hair algae. They do eat bubble algae like someone above said.
Here is what I did to get rid of my hair algae in my 55 gallon - I bought the "Big Boy Reef Package" from this site and added a few more crabs (emeralds, sallylightfoot), shrimp (coral banded, peppermint) and a bicolor blennie. The blenie didn't do much since it went carpet surfing a week later, but the algae is now completely gone and hasn't come back.