Colonial disc-anemone.

Scientific name : Palythoa cf. vestitus

It is a colonial Zoanthidea, with polyps connected at the base by a stolon. Flattened oral disc (1-2 cm diameter), with green or brown radial lines. Knob like tentacles. Many Palythoa species accumulate in the internal organs a very powerful toxin, and for this reason they have practucally no natural predators.
Many Palythoa species (sometimes named Protopalythoa) have a similar aspect, with separate polyps, and are very difficult to identify. Protopalythoa tuberculosa is identified by the thick leathery colonial tissue amongst the polyps.