What do you mean by calling a woman a ghost?

Qiannv Qiannv refers to a "beautiful girl".

So "ghost story" means a ghost with the appearance of a beautiful girl.

If someone describes a girl as a ghost, I think ta probably means that this girl is beautiful and has a ghost-like melancholy in temperament-usually called "gloomy beauty". Anyway, it won't be derogatory (unless the person who said this doesn't know what a beautiful girl is, only knows that ghosts are ghosts)