The head is protected by a large cephalic shield which extends as far as the central body where it forms a noticeable pointed protrusion.

[3] Other species of sea slug with which it could be confused include Aglaja tricolorata and Melanochlamys wildpretii; the former is tan with more-regularly arranged white spots, no cephalic shield peak and no orange and blue lines round the body; the latter has a more uniform orange-brown colour, no cephalic shield peak, and a slender white line that outlines the parapodia and crosses the mid-back.

The diet of Philinopsis depicta has not been precisely ascertained, but stomach contents have been shown to include Bulla striata and Haminoea species, it probably eats other sea slugs, and it has eaten Elysia timida in captivity.

There are no jaws, grinding plates in the stomach, or radula, so it is thought the sea slug protrudes its foregut and sucks up its prey whole; shells or undigested fragments are ejected.

When the eggs hatch after a few days, the veliger larvae are planktonic for a while, before settling on the seabed and undergoing metamorphosis into juveniles.