Condensation. It's normal. Don't worry about it. Replace the muffler & tailpipe & it'll be clear water unless you're running rich, then it'll be black water. It's the same reason you see vapor coming out of the tailpipe in cool weather that goes away after the vehicle warms up. The condensation in the muffler is boiling out. Condensation is also the main reason mufflers rust out.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.