I thought it would have been a broth

no I was asking where it’s from

It’s not a broth, because broths are made from boiling stuff in water until it takes on some of their flavor. That’s not how milk happens.

And it’s not a beverage, because a beverage is something you drink.

While a sauce is a liquid served with a food to add moisture and flavor, that’s clearly what’s going on here.

(But only because Grover put the milk on himself. If he’d brought out a bowl of dry cereal and a container of milk for Mr. Johnson to add himself, then it’d be a condiment.)

On the other hand, if Mr. Johnson has milk left in the bowl after eating the cereal, and he drinks it, that milk is a beverage.