This means that he has a severe respiritory infection and that he will DIE unless you get him to a vet promptly! Respiratory infections are common in tropical snakes, like Ball Pythons, that are kept too cool, or that have been exposed to other sick snakes, since many of these infections are highly contagious between animals. IF caught in time, they can be cleared up by injections of antibiotics, such as Amakacin or Baytril, which are only available through an experienced reptile vet, but it's also important to raise the temperature in the cage to the optimum requirement and keep the cage spotlessly clean. Quite often, though, by the time the infection reaches the point of mucous coming from the mouth or nose, it's too late.