After searching and searching for food for the purple nudibranch that is commonly sold at LPS's and wrong fully advised I finally found if on a sea slug forum. Most LPS's sell them and say they eat algae and a fewer smarter ones said they eat any sponge but after owning one with a sponge that he didn't eat obviously doesn't eat just any sponge. Most people said they are not ment for aquarium but I found an something sea slug forum that they eat 2 different type of family's of sponges. Hopefully this will feed them so I can have in my aquarium. Dysidea or Aplysilla sponges. Heres the web site.
http://www.seaslugforum.net/message/1755
Now I just have to find one of these sponge to test it