This is a "Reef Mirromy", a free species created by Iggi. I grabbed one of the free linearts there in order to mess around and come up with a design. I'm pretty happy with it.

Species, lines © Mirromy (FA)
Colours © Puncia

Oh, and it's based on a very pretty sea slug: http://i.telegraph.co.uk/multimedia/archive/01520/sea-slug-1-uwg_1520134i.jpg