I've never seen one before! Here on a 2.5:1 magnification! So small they are...

Also known as Crustaceans, Brachyura Crabs, True Crabs, Spider Crabs, Imitator Crabs, Xeno Crab, Tuberculatus Xeno Crab, Black Coral Xeno Crab, Whip Coral Spider Crab, Gorgonian Crab, Decorator Crab and Soft Coral Crab.

Found on coral and rocky reefs usually attached onto the Seawhips - Cirripathes or Stichopathes spp.
The crab will often take flesh from its host to use as camouflage or cover itself in algae again for camouflage!
They feed on plankton.
Length - 2cm
Widespread Indo-West Pacific

Brachyura crabs are considered true crabs, they have a short abdomen, 4 walking legs and a pair of clawed arms at the front.
Many thousands of species live worldwide in sea water, fresh water and on land.