Micro Decorator or Mime Crab
Mime crabs are peaceful algae eating crabs of the Epialtid family that will move around eating algae and blending in to avoid predation. Each species is suited for its environment. Many Mime Crabs in Florida are unnamed species, as they have been overlooked so far by naturalists. In your aquarium they will behave well, and eat just about any algae put in front of them, but they are small (often tiny) and slow cleaners and not cost effective. This "item" will include members of the following genera, all of which have the same behavior in captivity:
Pictured in the main item description is Epialtus coletti, the Heart Mime Crab.