Apparently Lettuce seaslugs are seaslugs and not technically "nudibranchs." To a biology nerd apparently there's a difference but I'm afraid I am not that particular biology nerd.
Good luck, with the ample food supply he might do well. I never had much luck with these guys though (couldn't find them after 3 days, and after that, maybe only a fleeting glimpse of a body-less slug head zipping around the tank.
Weird..)
Let us know how he does though! I hope he does the trick.