Emerald CrabHow to Care for Emerald Crabs written by Dave Burr The Emerald Crab is a superb algae eater, and probably one of the very best at cleaning eating bubble algae. It is very useful in keeping reef aquariums clean. Behavior: Large specimens can be harmful to small fish and invertebrates although this is less likely if there is sufficient algae for the crab to eat. Growth: Maintaining proper Ca, Alk, & Mg levels is important for growth of their shells.