Hidden Cup Coral - Phyllangia americana
If you have ever acquired liverock that came from Florida, odds are it had a few individual polyps that extended short tentacles. Those are called Cup Corals, and don't live long in our aquariums due to a lack of a regular food source. Generally I would just advise that you enjoy them for as long as they last. Cup Corals are not sold to hobbyists, they are a free hitchhiker coral that come on rock sold in the aquarium trade.