Umdrella snail or Umbraculum umbraculum.

These whopper sea slugs are a weird one!
They are basically a nudibranch with a hard top that gets all kinds of life growing on it, though they do bury to clear the growths, its still not enough, there is always some on these guys.
As per all nudibranch/snails they eat what you like in reef tanks so not reef safe, they eat sponges.
buried








As with all life not taken and any rocks or reef, put it back where you got it from.