ID

[Enigma]

It is Glaucus atlanticus.

Despite their often dubious content, here's the wiki article:
http://en.wikipedia.org/wiki/Glaucus_atlanticus

Here is the WoRMS entry:
http://www.marinespecies.org/aphia.p...ails&id=140022

Edit:
According to this article (http://www.seaslugforum.net/find/glauatla) they feed almost exclusively on Physalia (hydroids). It is probably not a good choice for an aquarium, as food could be in short supply (unless one has a booming population of colonial hydroids . . . though I think these guys prefer the Portuguese Man of War).