If it's a lettuce nudi, you're golden (just watch out for intakes/overflows - they LOVE those)...
If it is where I'm thinking of, just tell one of them what you found out, and that you want to return it... They absolutely WILL let you.
Granted, the ultimate responsbility for deciding to take this animal home WAS yours - so they may not offer much more than in-store credit and be justified, but they should be willing to work with you on some level... Especially if they gave weak advice on it (granted, as much your job to know about it as it is theres... Pet Store people are good generalists, not specialists... Can't know the details on every animal they carry, but YOU SHOULD know the details on every animal you buy)
Phyllidiidae are bad pets for most everybody but species tanks and if you have actually narrowed down the diet (some of them you can, but rare at best) and can provide it (even more rare!).
Glad you've been more controlled rather than unloadig on the store - great way to handle this, rather than blast from the hip!
You shouldn't have a problem... If you do, please let the board know.
