Hmm, this is a difficult question actually. There are over 3000 known species of Nudibranch. Very few are collected in the aquarium trade, however, many arrive as hitchhikers throughout the process. I guess most of the Nudi's collected for sale in the hobby are considered "reef safe", assuming the basic definition as not eating corals/"desirable" pieces. That being said, ALL Nudi's are carnivores. They are very, very specific carnivores, at that. Any slug that isn't a carnivore is not a Nudibranch. The lettuce slug that most hobbiest encounter is Elysia Cripata. They can be kept with success, but they have very specific needs as well. The purple nudi you mentioned could be one of many possible slugs, but I'll guess and say it's Hypselodoris bullocki. This slug only feeds on a species of sponge. Which exact species, we don't know. We do know that they will starve without it and we can't provide them with it. I can probably find a pic for you to confirm this is the same slug your interested in, if it helps. Nud's are extremely fascinating and beautiful animals and I hate being the one to tell people, that as a rule, Nudibranches should not be collected...