Umbraculum umbraculum (Lightfoot, 1786)
umbrella shell
Umbraculum umbraculum

sub-classs Opisthobranchia > order Umbraculida > family Umbraculidae

At first glance the umbrella shell, Umbraculum umbraculum, could be mistaken for for an egg mass. The animal here is attached to an overgrown rock wall. It has a warty body and a small flat shell, which is too small for the animal to withdraw into. In this photo, the shell is overgrown with organisms similar to those on the wall. Umbraculum umbraculum has a light brown to bluish body and a single, plumed gill on the right side of the body.

Distribution =  widespread Indo-west Pacific
Max size =  shell length to 15 cms

more info at Sea Slug Forum     more pics at nudipixel