Pseudocorynactis cribbeorum aka Carribean Mushroom, non-photosynthetic mushroom anemone from the carribean. They are really cool looking!!
Awesome ID!! 8) We have a couple of these in our tank, tucked into a cave. They do not seem to bother anything at all, and stay rather small/solitary, but keep in mind that these are known to have a very nasty sting, and will/can potentially kill small fish, snails, etc.