Flabellid Coral (Javania sp.?)
These corals in the family Flabellidae (possibly a species of Javania) were found encrusting the 200 meter platform of the Slope Base Shallow Profiler mooring, which sits above the continental slope at 2900 meters. These are a diverse group of solitary, deep-water corals that do not contain photosynthetic zooxanthellae. Corals in this family tend to be flattened, with a short, thick peduncle and a long elliptical, slightly curved calyx, and narrow, closely-packed septa. They likely arrived on the platform as larvae, found a hard surface to attach to on the profiler platform, and grew into polyps that feed on organic material falling down from the surface (or produced by other animals that grow on the platform).
References:
https://en.wikipedia.org/wiki/Flabellidae