That looks like
Phyllangia americana or one of the other many Caribbean species known as "cup coral". Non-photosynthetic, needs small meaty foods to live. I have some of those on my LR as well in my sun coral tank and I use them to catch whatever the sun corals miss. The skeletons on mine are a little smaller than a dime, and the polyps when open can be about 1 1/2" wide. Pretty, aren't they? Like very long-tentacled transparent sun corals.
Edit: Or, well, it could be Aiptasia
If it doesn't have an obvious hard skeleton when closed. But I think I can make one out in the picture.