white mouth

Hello I am bit concerned about my purple tang he has a white mouth. I attached some photos all other fish appear fine, all fish are eating well. I was hoping I could get some feed back and this would be something obvious to a more experienced fish keeper. Thank you in advance!

I have had this guy in QT for about 6 weeks and can not get him healed up. Did observation for 2 weeks and got him used to QT life and he is eating great. Good weight and color. Dosed Prazi (normal procedure) and was feeding Metroplex/focus for some other fish that had worms. He was getting it...