I've noticed these Cup Corals in my display tank, but they tend to only come in on newly shipped live rock. People that order from Tampa Bay Saltwater get LR with these little guys scattered sporadically in their order. They are filter feeders, and usually don't last too long in captivity. Note these pictured are quite small, especially when you compare them to the hitchhiker feather dusters to the right. Reef-safe
This Cup Coral is about 1" wide. I discovered it deep within the rockwork of my reef tank. It is almost invisible during the daytime, but late at night with a flashlight I could see it quite easily. The tricky part was getting a picture. ;) It looks very similar to a Suncoral polyp; note the tentacles are clear with tiny speckles.