thank you for your query.
i think most probably you're experiencing a rare condition known as a 'patent urachus' or a 'urachal cyst'.
This is a fine biological duct or a fluid-filled swelling between the top of the bladder and the belly button (the umbilicus
The duct has a function in the embryo but then normally closes-up to become a mere ligament, which is an empty cord of scar-like tissue.
Very rarely it does not heal and can present later in life as a swelling, perhaps discharging urine or mucus from a cyst within an open (patent) urachus.
other common causes can be-
-lump in the belly button
i would advise you to consult a GP,who will take a sample of the fluid and send it for analysis and also do a complete physical examination to rule out other causes.if confirmed the cyst can be fixed with surgery.
wishing you good health