Elysia diomedia
The genus of this nudibranch was changed from Tridachiella to Elysia. Like other Elysia the ruffles coming off the back (which contribute to the common name of "Mexican Dancer") are charecteristic of the genus. Most Elysia eat algaes. This species is found in the Pacific. It has a sister species in the Caribbean: the Elysia crispata.
In the following photograph, there are two E. diomedea. You can see how the color patterns on the edge of the frills of the nudibranch can actually function as camouflage. In the first photo, you can see how colorful they are upclose, but move away a little and it blends into the background.
