Is it cannibalism if my Nuzleaf eats greens? He doesn’t eat nuts though.

This is actually an excellent question with an awkwardly simple answer: no. Just as eating beef doesn’t automatically make you a cannibal, allowing your nuzleaf to eat greens doesn’t make him one. There are plenty of species of plants, so a plant eating another plant is really more or less akin to an animal eating another animal, if we oversimplify things.

However, technically speaking, you would be more of a cannibal than your nuzleaf. Although plant pokémon bear striking similarities to actual plants on a cellular level, they’re not even in the same phylogenic kingdom (as opposed to us humans and cows, which are). So, yes, there’s nothing wrong with nuzleaf eating plants.