Many fish can be resistant to ich infestations. Some tangs can not. I have not been able to keep chevron tangs in tanks with ich and manage it as I have with zebrasoma tangs (purples, sailfins, yellows), naso tangs, and hippo tangs. I have heard of people succeeding though.
I wouldn't risk it I would get it in a hospital tank with .5 ppm cupramine ASAP for 4 weeks. Do not use prime or amquel or any nitrite nitrate or ammoni detoxifiers as these used in conjunction with cupramine will render the copper toxic.