Volkswagen Passat water tank!! (brown gunk inside the water tank)

greetings to all that can help!!

i drive a vw passat and i have found brown gunk in my water tank. it is still running fine, the brown stuff i cant be sure of what it is but i am worried that it maybe something more serious than i first imagined.

i have been reading that it maybe oil from the coolent system but i dont understand how or why?


if there is anyone that can help that would be really helpfull to me and also solution before i go to the macanics.

thanks