A plate refers to any number of corals that form flat like a plate. Most frequently it refers to heliofungai sp, but I have also heard of the turbinaria and pagoda corals referred to as plates as well as monti's. It may be a turbinaria or pagoda, but these typically are hard stonies that have a skeleton, rather than encrusting a rock. Then again, maybe the rock you have is not just rock, but a pagoda coral that nearly died.
Hopefully, one of our moderators or someone with lots of reefing experience will give you a definitive answer soon.