Decorator Crab

Carpilius corallinusTrue Crabs - Order Decapoda

The fact that you can cannot see the crabs under their "decorations" proves what a great advantage these crabs have over others.
The first crab has decorated himself with living Spiny Seaweed. 
The second crab, with the lime green under-belly, has a variety odd bio-matter attached. 
Number three has decorated himself with some blue Heavenly Sponge.
Number four is a species of Neck Crab. They have a preference for decorating with hydroids. This one is cultivating a Christmas Tree Hydroids.
The fifth crab is tiny and has flat tunicates residing on his back. 
Number six is carrying a piece of pink sponge to hide under.
The  seventh crab would never have been spotted if he hadn't been walking across the top of a rock. His choice for camouflage is quite a heave burden. He's using pieces of rocks and discarded snail shells. When he stopped after climbing down from the wall, he spent several minutes re-arranging his disguise. 
Number eight may or may not be a decorator crab. He has Cyanobacteria growing on his carapace. The crab may simply be a carrier and did not intentionally decorate himself with it.

Decorator crabs are known to decorate themselves to avoid predators. This is a highly specialized behavior.  Crabs will decorated to hide, but will also decorate themselves with toxic materials to dissuade a would be predator from dining on him.  If moved from one environment to another, they will quickly "change their outfit" to one more like the new environment. Most decorator crabs found here are small, one-half inch to 1 inch and in  shallow water. They are True Crabs.