flabellina affinis

(photo by Rai Fernandez)

This particular sea slug (Flabellina affinis) is found in the Mediterranean and grows to about 50mm long. Sea slug  is a common name used for several different groups of saltwater snails that either lack a shell or have only an internal shell, in other words this name is used for various lineages of marine gastropod mollusks that are either not conchiferous (shell-bearing) or appear not to be.

(Source)