April 16, 2002
From: Don Pelino
I found this sea slug in my aquarium's refugium. it's about 1/4" to 1/2" long, vivid bright lime green and looks to have either a white stripe or white markings on the back. It also appears to have a tail/foot that is somewhat flat or blade shaped although that could just look that way due to muscular movement. I'm guessing it came from Live Rock or Caulerpa from the Gulf of Mexico on the Florida coast as all of the LR in the refugium is Florida aquaculture and the Caulerpa came from Florida as well. Some from the Tampa, Florida area and some from Dade City, Florida. I don't believe any of it came from the Atlantic coast of Florida.
Pelino, D., 2002 (Apr 16) Lime green sea slug. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/6669
Your slug is a species of Oxynoe, and if it is from Florida then I guess it is Oxynoe antillarum. This species feeds on Caulerpa so as you guessed, this is the most likely way it got into your aquarium. If you don't mind it eating a bit of your Caulerpa, it would be an interesting addition to your aquarium. Have a look at my comments on Matt's message for some more information on the species.
Mating Oxynoe antillarum
From: Lureen Ferretti, September 1, 2009
Oxynoe antillarum from Jamaica
From: Ross W. Gundersen, October 14, 2003
Oxynoe antillarum from Belize
From: Dave Behrens, October 19, 2002
Unknown stowaway on Caulerpa
From: James Stein, September 3, 2002
This guy suddenly appeared in my aquarium!
From: Matt Galvin, October 7, 2000