Does it look anything like this? If you have a young male (4-7 months) you might have just been flashed. They do tend to sometimes do this but it should retract after a while. If it does not retract for a long period of time (hours, days) you can try and soak it in warm water to get it back in. If it stays out for a long time, it can dry up and actually severely harm your Chameleon. If you try soaking it and problem persists, you'll need to contact a Reptile Veterinarian ASAP.
Picture is a little blurry...but if it looks anything like this...
It's likely a sperm plug. You'll see these in males as they're maturing, and they'll work their way out, usually when they poop. If it is a sperm plug, there's nothing to worry about.
It looks like it might be a hemipene. How long has it been out? It needs to be kept moist. KY jelly works to lubricate and keep it from drying out. If there is any risk of him damaging it on branches, put him in a box on moist paper towels slathered in KY jelly. See the post from Wowbango. Not all hemepenes that are prolapsed look like Wowbango's picture.
Picture is a little blurry...but if it looks anything like this...
It's likely a sperm plug. You'll see these in males as they're maturing, and they'll work their way out, usually when they poop. If it is a sperm plug, there's nothing to worry about.