There seem to be a few Spanish expressions about milk (leche).
apart from estar de mala leche - to be in a foul mood
there is also:-
darse una leche - to come a cropper
¡te voy a dar una leche! - I'll thump you
¡qué leche tienes! - you lucky devil
hace un calor de la leche - it's bloody hot
ser la leche - to be unbelievable
ir a toda leche - to go like the clappers
poner a algn de mala leche - to p*** someone off
tener mala leche - a nasty piece of work
PS I wonder what the Spanish would make of "the milk of human kindness"?
"la leche de la amabilidad humana" is my clumsy translation.