Dear Bill, I was given these two pictures taken by a local diver, and I have not the slightest idea what is it. My best guess is a slug, possibly related to cerasinu Ceratosoma sinuata (where I saw in your site green and speckle
This is not a specially designed birthday cake. It is not a palette too – it is called Clown Nudibranch. This bright and uniquely colored sea slug can be found in the intertidal zones of temperate southern Australia and northern New Zealand.