Actually, there is no such thing as a Lettuce Nudi, they are slugs, but it is a common mistake. Nudibranchs and slugs are two completely different animals that look kind of alike. They do not eat algae except bryopsis. They don't actually "eat" anything, but they suck the chloroplasts out of bryopsis. I raise the things. Here is an article I wrote a few years ago on them if you are interested.
Take care.
Paul
JoM Article: Elysia crispata Sea Slug Spawning in a 100 Gallon Reef Aquarium, by Paul Baldassano