Has it always been there? It's not unusual for chameleons to be lighter around the lips and nose.
It might be due to excess calcium. It's not a problem, it's just the animal taking care of itself. You can try cutting back on the calcium a bit and see if that clears it up. You can do that by either skipping more days or pulling out a couple of feeders to dust. Give that at least 10 days to take effect!
Pictures would still be a good idea because it might be something completely different and the experts here might be able to recognize it.
The fact that chameleons vent out excess calcium is really cool! As those of us who have Guinea Pigs can tell you, they send excess calcium to their kidneys and bladders in the form of stones....that's a problem.