Decorator Crab or Spongy Decorator Crab, Marcocoeloma trispinosum is a crab that we find on a more common basis compared to the red decorator crab. . They get their name from attaching a variety of things to their body to blend in with their surroundings. They might start putting zoanthids etc on their back which makes them not reef safe in our opinion. As far as food, they can be classified as a scavenger but it is not clear if they would help with any algae problems.
We will try to send you a specimen with decoration on it but they change so much that it is not always possible. They are between 1 to 2 inches in size.