Looks to be a Cycloseris coral, which is a "plate" coral and will have a skeletal disc attached to the rock. The extension of its marginal tissue will hide its "plate".
Hey Charlesr1958...Went to your page, and it does look like that. Haven't seen a skeletal disc attached to the rock though. It's about the size of a quarter. I'll try and spot feed it today.
I collected the one shown when it was about quarter sized also, with its marginal tissue extended, the actual "plate" was about dime sized at most. If you gently touch it, it should retract and its skeltal features should become more apparant. At times, if the light is strong enough, you can see through its tissue and make out the shape of its "plate". They are very active feeders and do much better with frequent feeding.