The Lactiferous duct is not something like milk or made of milk or akin to milk etc. but it's simply transporting milk. If the Cs would have used the word plantiferous then I could maybe understand it as funny way to say 'vegetarian' (one whose digestive tract carries plants). But since it was used by Laura it seems to me she simply meant plant-like as in biological or physiological. Since the C's said 'grays' I imagine these entities to be pretty much humanoid or at least more like animals than plants.