Florida cup coral?
This looks to me like a Florida hidden cup coral. They are frequently mistaken for glass anemones, but they do have a skeleton unlike the anemone. Sure way to tell is to gently poke it with something soft (like a brush) and see if there is a "crater" left when it hides. A glass anemone will hide itself completely into the rock, but the cup coral cannot hide its skeleton.
I have quite a few that came with aquacultured LR from Florida, and your rock seems to have similar algae growth on it.
These corals require no light (they dont have any symbiotic algae), they just catch whatever they can from the water. You can feed it some mysid shrimp, bits of fish or whatever else meaty you have in the freezer, and it will grow to a nice quarter size animal. I like them a lot. Large specimens have a nice greenish hue.