I think when Innovator mentioned about corralites, he meant that he thought he could see coral skeltons under the white stuff. (At least that's what I *think* he meant! He'll let us know soon!) I think it was just the rounded surface of your rock that was making it look like that.
The picture you've attached is either a dead coral skeleton, or a live one that's really unhappy. Can't really tell for sure with the picture. Time will tell if it's alive. Either way, it's not bad to have in the tank.
Did the mystery slime come off in a sheet, or did it just kind of crumble and come off in chunks?
[Edit: OK... after looking at the pictures more, I'm confused. Did you take the rock out to clean it and put it back in a different position? Was that white slime covering the coral skeleton that is in the center of the last picture you took? ]