Blue Club Tunicate

Found in colonies, in caves, crevices, and between coral branches, in shallow waters of fringing reefs.
They feed on detritus and suspended zooplankton.
Often fed upon by Nembrotha Sea Slug.

Ascidian, also known as tunicates because their bodies are covered by a "tunic" a tough cellulose-like covering.
The other common name is sea squirt because when threatened they sometimes eject a jet of water.