Photographer: Paul Osmond
(see this users gallery)
This is the Chromodorididae Nudibranch Hypselodoris whitei moving across the rich, dark, ocean sand floor at the Tulamben Slopes dive site in Tulamben Bay in Bali, Indonesia.
There are a number of similar colored species worldwide - specifically Hypselodoris maridadilus (which lacks the white tips on their rhinophores) and Hypselodoris emma (which has only three purple stripes).
Tulamben Slopes is one of the premier muck diving sites in the world, featuring black sand slopes that are some of the richest in the world.