While the engine is running? If so I marvel it didn't ignite in the exhaust manifold. Sure its not water? If so its one of the products of the Oxygen Hydrocarbon reaction. The other is Carbon dioxide. Of which 2/3 of the mass is from Oxygen not carbon.What would cause gas to come out the exhaust?
If its while the engine is running, the cause could be a dead cylinder or a mixture so rich as to be condensing back into a liquid in the pipes. Something I have never seen. But surely has the potential to attract all the neighbors when the fire department arrives.