No research appears to have been done on feeding and growth in west-coast cup corals. Food is likely to consist of small planktonic crustaceans, worms, and various invertebrate larvae. The tentacles are well-endowed with nematocyst batteries (as evidenced by the small white spots in the photo) and these are used to capture the live prey and pass it to the mouth. The food is digested in the gastrovascular cavity and undigested wastes released via the mouth to the outside.

Cup coral Balanophyllia elegans egesting unwanted food residue
consisting, in this case, possibly of some crustacean bits 4.5X