SOLVED: "sex powder"

Erotic novella in an anthology
H is an alien who is multicolored (he might be able to make himself look human)
This scene I clearly remember: the H secretes some kind of powder, all over his body, during sex. The h is compulsive about licking the powder. When she goes into the bathroom right after sex, because she is really thirsty due to the powder, she catches a glimpse of herself in the mirror. She has changed colors (I don't remember the color, I think blue or purple) which means she has been changed into an alien like the H.