Here's an Oval Mushroom, or Razor, Coral....

We see these corals from time to time while on dives. They're not overly common and many divers don't notice them. The species name is Fungia scutaria. They're interesting in that they are not sessile, they are unattached and can move around a bit to where they get the conditions they want. Divers occasionally notice them when their tentacles are retracted and invariably think they're dead, I really discourage touching them as unless it's bone white, it's alive. Another reason not to touch the corals, dead or alive, is that Hawaii has a law making possession of many corals illegal, so it's not a good idea even if it's passed on the the great reef in the sky.

This is the first photo I've got of them with their tentacles out quite nicely. Back in my aquarium days we used to keep these in our reef tanks, they'd really come out nicely under the proper lighting. I'm not sure where the source was, I suspect the Philippines and other areas south of here, but some of them had remarkable coloration.