Cats sometimes just like chewing on green stuff. A little bit shouldn't hurt them as long as it isn't something specifically poisonous to cats! It's natural for carnivores to supplement their diets with grasses and leaves to obtain some small amounts of fibre. And sometimes it's just exploratory behaviour. They don't need a lot, but they do need some in their diet, so no worries!
*just noticed the bone comment! I would avoid cooked bone since it can splinter and cause problems. But if you clean off the gristle, a small amount as a treat should be fine.