I may or may not have the answer of your disappearing nudibranchs as I have not seen what they look like at J&L. On Dr Rons board they were talking about a creature commonly called a lettuce nudibranch that is actually a sacoglossan slug. According to the good Dr. once they lay eggs they will stop eating as much and die. Sorry about the news if that was what they were. The thread can be found
here. I know that they are for sale here in Calgary and the fact that they can store sperm for a few months lay eggs and then die on me has made me decide to avoid them.
-Danny