Nudis are a  big-time herbivore.  Don't purchase these unless your tank has algae or your willing to feed them algae.  They love hair algae and they are great for going in there and clearing it up for you.  They are very popular, reef safe, sea slugs that eat algae from the walls and rock surfaces of your aquarium.
Once they run out of algae they have to be supplemented with more or they will not last long. They can eat a ton of algae, synthetic algae because it is one of the things that gives them all their energy to go around and do more of it.
Reef Safe: Definitely, they do not mess with any corals, they would not mess with any of your inverts or anything like that.
The Lettuce Nudibranch will forage for algae throughout the aquarium. It gains energy through photosynthesis and through the algae it eats.