Sea buttons or playthings of unknown name. I have many of these. Some bluish, green,tan or ugly brown. Rather depends on the light. Can be invasive and grow over other corals like scrolling montiporas. They also release alot of slime when taken out of the water so be wary of palytoxins.
Palyathoa's contain a toxin that can be lethal and no one knows just which ones contain the palytoxin that could make you very sick or worse. It's been said that the more invasive ones are more suspect in being toxic. Always wear gloves and glasses when handling these to prevent any problems especially if you have a cut on your hands or they are cracked and dried from playing in the saltwater so much Also if you rinse things down the kitchen/bathroom sink be sure to soap and rinse well so others don't get contaminated from them. Some people report getting sick from the slime of them and some have gotten very bad reactions from getting it in their eyes. Error on the cautious side when handling these please. I know it sounds a bit scarey and over the top cautious but better that then a trip to the ER or worse.