That’s a parrotfish ready to take a bite of algae growing on dead coral. Parrotfish teeth are incredibly strong. The dead coral that this “excavator” bites off with the algae gets ground up by a second set of teeth in the fish’s throat and is pooped out as sand!
Parrotfish are great at keeping the ocean floor clean of algae, so that corals can grow. However, it’s recently been discovered that they can be a problem when scientists attempt to restore reefs by putting laboratory-raised corals into the ocean.