It looks like it can be an abscess. Sometimes they start out as a sinus infection, and the infection spreads and forms an abscess like that. You should take him/her to a good reptile vet. They'll probably have to clean it out and prescribe some antibiotics.
Make sure the vet cleans it out thoroughly because chameleon pus is thick and just giving it antibiotics won't get rid of the access. Have a culture and sensitivity test done so that the chameleon gets the right antibiotic to get rid of the infection.