Probably just condensation collected in the muffler/tailpipe. Common when the humidity is up & temperature is cool. Some mufflers have a drain hole, which gets stopped up with rust so the moisture blows out of the tailpipe.
Water is a natural by-product of the internal combustion process so its not abnormal to see some water in the form of vapor and even small drops at the tailpipe especially in cool damp weather when the water in the exhaust condenses on the exhaust pipe walls until they have reached full operating temperature.
However, if condensation builds up in the fuel tank and gets drawn into the fuel this can cause poor performance and corrode the fuel system from the inside out. Heet or other fuel system drier can help eliminate the effects of this situation. Just add a bottle next time you fill up. I use a bottle per tank in cold months to prevent the condensation from forming ice in the system.