Fungia plate coral. I have 2 plates and a tongue that came off live rock and zoa rocks like yours. If you're lucky, it will grow to about the size of a half dollar and detach, then a new one will grow from the "stalk". I got about half a dozen tongue coral that way from a piece of live rock...still have one after more than 10 years. Yours plate looks like it is going to be nicer than mine.