It is the skeleton of a coral (sponges don't really have skeletons like this) It looks dead from what I can tell.
Is this Florida rock by chance? Looks like it could be the skeleton of Siderastrea or similar.
Could very easily be an ecrusting type of sponge. IMO, it is and alive. If it is and its dead it will rot away.
But also could be anything. Could be a live or dead coral too.
"" Really need a better photo. ""