It is important to keep in mind that snakes have very 'slow' immune responses. The symptoms might not be directly related to the actual course of the infection. It can take a while for the drugs to take effect and for the snakes immune system to mount a response and kill the pathogen. I would not recommend trying to remove the mucus from the snakes mouth. Usually the animal can eliminate the mucus on their own. Stick to the advice from the vet.