Washing doesn't do sh*t, except keeping you busy that is
If it's just the tail, there's nothing to worry about. If it's al over the body, it's most likely an adrenal problem. Or in case of a Jill that's kept too long into season it's often (literally translated from Dutch to English) bonemarrowdepression. Both diseasescan kill if left untreated. But as I said, as long as it's just the tail and not the ass, it's nothing to worry about. Mostly this only occurs in the summer. With some ferrets each summer, with others only one or two summers.