While I wasn't present on your particular field trip; I think I know to which fish you are referring. The California Sheep head, like other wrasses all start out female and following specific, and yet to be fully understood environmental cues, become males later in life. This is in contrast to the anemone fish (the group to which "Nemo" belongs). In this group, the juveniles are all male and it is thought that through the development of a dominance hierarchy, the most territorial individual becomes female, which in most species is clearly the largest of a male / female pair.

Regarding your octopus question: Octopus are remarkably agile and, as a result, are notorious aquarium escape artists. Unfortunately I do not know what the particular octopus in question ate on its incredible journey away from its home aquarium.

The next time you have the opportunity to visit the REEF, you might ask the tour guides this same question.