Had a lots of debates on this one, few thought LPS Bubble Coral or Frogspawn but it has no skeleton, but most suggestions have been inflated Ricordea Yuma and def a mushroom species....just a rather bizarre one. It's my faviorite coral in the tank and getting to about 5 inches across now.
The last image is when I first got it and there was a second I swapped for some sps.