Soggifier
I have started my day with a bowl of crunchy cereal drowning in a white liquid for more than a half-century. In all that time, I have only called it milk.
Co-gredient?
Sauce?
Broth?
WTF?
Another vote for Milk.
Milk.
First!
I think the tradition of putting grains in a bowl with milk over them came from European immigrants who put bread in their coffee (perhaps because it was dried out). This article (.pdf) from the New York times in 1987 indicates that it might also have been the origin of Postum as a child’s version of coffee.
And, in fact, some people on a farm in northern Italy once gave me for breakfast a large mug of coffee with milk that had crouon-like pieces of hard bread already in it.
Icky.
It’s just milk. Even when it turns interesting colors from the sugar-laden cereal floating in it.
It’s milk in a bowl.
Milk.
WTF? It’s still milk, or course. Stoner question.
Milk. What the hell kind of weird question is this?
Milk. Or ingredient (of the dish “cereal with milk”).
To clarify, are we trying to decide if there’s a name change for the milk itself when used for this purpose, or is the question more like “what do you call a liquid specifically used for soaking other food with?”
Are we talkin’ cow milk? Goat milk? Mother’s milk?
It’s all just milk.
My cereal makes it’s own gravy!
wasted milk on ruined cereal.
Chocolate milk. 
Y’all ever have an “Irish Breakfast”?