Saba_20100324_2147-1-1

It's a nudibranch - a type of sea slug basically.

A nice photo with good visual interest from the main subject and the coral patterns. I have absolutely no experience with underwater or aquarium photography, but I do know that it is not easy subject matter to photograph.  The only suggestion I might add is to frame (or crop) just slightly differently to take full advantage of the interesting coral branch and use it as a compositional  "leading line" from the lower corner.