Plate coral

[Coralgurl]

I have what I think is a plate coral but I don't know where it came from. Found a weird shell like thing quite a while ago, checked it out, left it in the tank and noticed a few days later it was gone. Didn't think much of it. A week or so ago, saw what I thought was a mushroom tucked in some rocks, score....but it moved around, one day there, next day gone. I finally found it again, pulled it out and it's a hard base with spines on the top. So plate coral right? Put it back in tank in front on the sand, today it's moved back into the rocks. It's reddish and has tentacles. My camera is in my car so I can't get a pic, which I know would def help. Do plate corals move? Or is this something else?