Baby plate I bet.
LPS corals have an amazing rebound ability. That dead plate probably had some living tissue left in it and it slowly recovered. THis is not unusual, and I have seen other plate corals do this. Sometimes they even become baby factories, spitting off new ones pretty often. Must be some kind of survival mechanism.
I never hesitate to buy dying LPS corals in a fish store if they are drastically marked down because of this. I have bought an Acan Maxima, Bubble Coral, and a Blasto Welsi colony that were dying, and all have bounced back, and fairly rapidly.
Did the original plate die on you or did you pick it up from somewhere?