First off, It can be very dangerous to leave live mice and rats in the tank for long priods of time. They can and will attack you snake. Make sure you check thoroughly for bite marks.
Secondly, if the discharge is mucous, then your snake most likely has a Respiratory Infection and will need to go to a vet. You need to listen to see if your snake is wheezing, or keeping his head elevated alot. You may also need to check inside his mouth for alot of bubbles or mucous.
There is also a possibilty that you snake may have just drank some water and didn't swallow it all, but with the refusal to eat, discharge and lethargy, I would tend to suspect a R.I.
What type of heating and controlling set-up do you have?