PROBABLY a Zooanthid. It COULD be a playthoa if when its closed you see dark lines radiating out from the top center of the heads. If you dont see any lines, its a zoo
Does it have a stony skeleton or are the polyps connected at the base. Now that you mention it Mr M, it does look like a goniopora, at least in terms of color morph
Goniopora, when retracted, doesnt show its true skeletal structure. If it was truely "flat" it sounds like Goni to me, but if it had individual "mounds" that form polyps, Ski is probably correct, a Zoa or Paly, but I think its closer to Goniopora myself. Terrific colour. The site I linked to also has information on their general care.
Unfortunately it's not mine as I haven't got a marine tank at the moment. It's a picture I took at work. I picked it at the supplierspretty much because it looked good, but they didnt charge us for it so we didn't get the name on the invoice.
Might have to shift it to our display tank instead!