When a man wakes up in the morning, sometimes, he gets a boner, an erection, a hard on, whatever you want to call it. Sometimes, when a girl wakes up in the morning, she is wet, aroused, horny.
For men, it's called "Morning Wood."
It is called "Morning Mist" for girls because of the warm and wet feeling produced down below. Kind of like a mist.
Dude, I got Michelle horny before she fell asleep last night and she woke up with a morning mist!