If you imagine a shape and color, there is probably a nudibranch to match.
British Dictionary definitions for nudibranch
nudibranch
/ˈnjuːdɪˌbræŋk/
noun
1.
any marine gastropod of the order Nudibranchia, characterized by a shell-less, often beautifully coloured, body bearing external gills and other appendages Also called sea slug
Word Origin
C19: from nudi- + branche, from Latin branchia gills