Mushroom Coral.

Fungiidae (pronounced /fəŋˈɡiː.ɨdiː/) is a family of Cnidaria that contains the mushroom corals (sometimes known as disc corals, solitary corals or plate corals).

Characteristics

Species are generally solitary marine animals that are capable of benthic locomotion.
These corals often appear to be bleached or dead. In most genera, a single polyp emerges from the center of the skeleton to feed at night. Most species remain fully detached from the substrate in adulthood. Some are immobile as well as colonial.
Mushroom corals are also able to switch sex from male to female and back again