Red-throated Ascidian

Found singly, sometimes covered in algae, in shallow sheltered waters, but has been found deeper in harbours, on ship hulls and shipwrecks, over coral and rocky reefs. The shrimp Pontonia sibogae is often found living in the sac of the tunicate. Varies in colour.
They feed on 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.