Micromelo undata (Bruguiere, 1792)    syn?  Micromelo guamensis (Quoy & Gaimard, 1825)
wavy-lined bubbleshell
Micromelo undata

sub-classs Opisthobranchia > order Cephalaspidea > family Hydatinidae

Distribution =  widespread Indo-Pacific
Max size =  20-30mm

more info at Sea Slug Forum      more pics at nudipixel