My guess is some sort of mushroom coral, viewed side on, though as David said, to say with any degree of certainty, we'd need a better pic. Can you fish out the bit of rock it's on?
I tried, but when I do it retracts and is no longer visible. As you can see, it is growing on some sort of calcium structure. This is why I came to the conclusion that it was either an SPS or LPS coral. I may be wrong though.
Try fishing out the bit of rock it's on and putting it somewhere in the tank where it's easier to take a picture. It'll extend a while after you put it there and you should be able to get a better photo.
Do you have a macro function on your camera? That'll enable you to get a closer shot. Check your instructions to see whether you do.