Purple Ascidian

Found singly or in colonies, often in shallow waters, but has been found deeper on coral and rocky areas of reefs, and in sheltered waters of harbours and on shipwrecks.
They feed on detritus and suspended plankton.

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.