Crabs are an essential cleaner, but not all are safe for coral reefs.  There are many different species and they vary in size and structure.  Crabs are remarkable scavengers that are constantly removing algae and debris tucked deeply into coral reefs and rock reefs.  They do well in any environment and are generally peaceful.  Some Crabs, however, can be more aggressive, so it is best to allow them plenty of space and extra food when needed.