Discharge from the nipple can be very concerning and there are a few basics that you need to know.

If a discharge is found on one side, is clear or bloody and comes out without stimulation, then evaluation by a breast surgeon is needed. 

The breasts are created to produce milk therefore it is normal for fluid to come from the breasts. The white milky substance produced in the breasts that occurs when a woman is not breast feeding can be associated with other causes, therefore you need to be evaluated by your gynecologist if the white milky fluid comes form your breasts when you are not breast feeding.

If the fluid is green and only comes out of the nipple with stimulation, then is most likely the normal process of duct lubrication. It usually occurs in both breasts.

Nipple Discharge Causes

Non-cancerous causes                                                                           Cancerous causes

-Normal ductal fluid                                                                                   -Papillary Carcinoma

-Intraductal papilloma                                                                               -Ductal carcinoma insitu

   (a non cancerous growth in the duct)                                                    -Invasive ductal carcinoma

-Lactation

-Galactorrhea (milky discharge)

-Mastitis

-Infection

Nipple discharge needs to be evaluated with mammography, ultrasound,  possibly a breast MRI and possible a ductography. If the discharge is concerning to your physician, a biopsy of the duct will be needed.