Some kind of leather I think.

[Bonessurf]

Found this under a discarded coral skeleton. It is a attached with a stalk and is soft.

 

[Bonessurf]

A little better pic

 

[Bonessurf]

No ideas?

 

[angelf]

It looks like mushroom/fungus and it also looks like a carpet anemone very common caribbean ones but it most likely a mushroom.

 

[TomStav]

No idea, it's not a carpet anemone, it does look somewhat like a mushroom, but those arm things look like my kolt coral, but I haven't seen one grow flat like that, looks cool though...