Large Polyped Stoney Coral
From The Aquarium Wiki
Hard corals are in two categories, Large Polyp and Small Polyp corals.
Large Polyped Corals are typically sessile (remains in one place) with long sweeping tentacles. They come in a huge variety of shapes, sizes and colours. Often because of these tentacles, they need to be kept away from other sessile inhabitants so they do not sting and damage them.