This is actually a Guam nudibranch, or a Halgerda guahan. I have been looking for ages for the name of this thing. Finally found it on the Sea Slug Forum. I have several photos of these guys. I normally find them on yellow or orange encrusting sponge. As in this photo their ridge color will match the color of the sponge.
![316guamnudi-med.jpg](http://www.scubaclicks.com/galleries/data/504/316guamnudi-med.jpg)