It may just be condensation. Under the right conditions (relative humidity), the motor moves quite a bit of moisture through the cylinders along with the air. The main reason exhaust systems often rust from the inside out.
It may also be a leaky headgasket and you are pumping some coolant out the exhaust. Is your coolant level going down? Does the motor run rough especially at startup? If you are pumping coolant out the exhaust, let some drip on a paper towel and look at the color, if the moisture is tinted the same color as your antifreeze you know you have a head gasket problem for sure.