It is called a Lawnmower Blenny because it eats algae, but it also eats other fish food.
In spite of what the Wikipedia article says this fish can be really mean.
When I have tried to introduce others of its species into the tank they ended up beaten to death.
It also has the unfortunate tendency to try to bite other fish. Fortunately some bigger fish such as a Rabbitfish were able to defend themselves successfully, and then the biting has stopped.
Lawnmower blenny resting behind a patch of cabbage leather coral.
It loves to perch on or around corals for some reason.
On this picture the Lawnmower Blenny is sitting behind a patch of Cabbage Leather Coral.
It is a nearly indestructible coral that grows all over the place if it is allowed to grow unchecked.
It is quite common in the aquarium trade.
Further readings
Sinularia dura. on Saltcorner.
Cabbage Leather Coral on AquariumDomain.com.
The article Über Algae Eaters: The Lawnmower Blennies in the Tropical Fish Magazine.