Oh relax.
Beman's right, though. The animals' safety should always come first... Aside from the obvious possibility of the frog being maimed or eaten, grey tree frogs produce toxins on their skin that irritate mucous membranes (mouth, respiratory tract, eyes, etc.) that could be dangerous if the chameleon tries to eat it, or if the frog crawls/jumps onto your chameleon's face. If it's wild caught it could be harboring pathogens that can spread to your chameleon, too. "It hasn't happened yet, so it won't" isn't a great justification for cohabbing species like this.
Besides, there's always room for one more frog tank... just don't tell my boyfriend I said that.