crusty lips on a uromastyx

Question
Hi,
My Mali Uromastyx Lizard,Leo, has a white,hard, and thick crust around his mouth. He has had it for six months. We have brought him to two vets, and he has had a biopsy. The biopsy showed no sign of bacteria or fungus.  Leo is very active and has a great personality. His lips don't seem to bother him, but it bothers me. Have you heard of this before?"

Answer
Hello Elizabeth,

Do you have any pictures of him?
They tested him for a respiratory infection?  
What are the temperatures in the tank?  
I have heard of salt deposits being excreted through nostrils in iguanas, but around the mouth area.  Usually that is indicative of a respiratory infection.
Is he eating pretty well right now?
What do you feed him?

Tracie