This is a nudibranch. It is a type of sea snail, and is carnivorous. There are many different types and colors of nudibranchs, and many are brightly colored and fantastic. Some are cute. This nudibranch is cute.
The word "nudibranch" comes from Latin nudus meaning "naked", and Greek brankhia meaning "gills". The name is appropriate since the dorids (infraclass Anthobranchia) breathe through a branchial plume of bushy extremities on their back, rather than using gills. By contrast, on the back of the aeolids in infraclass Cladobranchia there are brightly colored sets of tentacles called cerata.