I'd likely say that is a build up of sperm deposits.
Here is a small blurb about them from the Chameleon Care Resources:
Sexually mature male chameleons form an exudate of sperm in the hemipenes. This is normally expelled during defecation – sometimes the male will evert one or both of his hemipenes to free the matter – and looks like white rubbery sticky string. This is normal and needs no intervention. In cases where these aren't expelled normally sperm or seminal plugs then occur and these look like a small lump of dried feces at the corner(s) of the vent. If these are not removed they continue to build up and may cause some discomfort. Soaking the chameleon in a warm bath will help soften the material and may help the chameleon to push them out on their own. Hardened or enlarged plugs should be seen by a veterinarian for removal as damage to sensitive tissue can occur.