The Green Emerald Crab is a good herbivore. They are reputed to eat valonia (bubble algae), although I've never personally witnessed this and there is still valonia in my tank. He does sit and pick in the rocks all day, as I caught him doing in this picture, and I have seen him eating hair algae. There have been reports of these crabs eating polyps and capturing small fish, although there is also confusion about whether they were actually mithrax species. Be cautious and observant when placing one in your tank.