Witch's milk is the milky secretion from the nipples of a newborn baby (boy or girl) that are a result of maternal hormones (especially prolactin) acquired just before delivery.
This is a benign condition and symptoms should disappear after a few days. If the condition persists, see a doctor.
Nipple discharge from a non-infant is NOT called witch's milk. In the absence of an obvious precipitant (such as recent childbirth), it should still be investigated.