If his mouth is filled with mucus then it sounds like he has a respiratory infection and you will need to get him to a vet. What do you mean by a bubble? Can you post some pictures of your chameleon?
If it is a RI you can use Reptaid if you have any on hand..if not I suggest a Vet App. no matter what..there could be something else going on that you cannot see..Good Luck
is his mouth swollen? When he gaped does it look inflamed? Could also be mouth rot. you should see a vet asap because mouth rot and uri are both very serious and can be deadly if left untreated.