That is Head and Lateral Line Erosion [HLLE]. It is not contagious and it is reversible.
You will want to immediately add a grounding probe to your tank, and any sump, etc., connected to the tank. What are your water readings, including nitrates and phosphates? What type of tank do you have this fish in? Reef, FO? Size of tank? What are you feeding this tang?
Take some time to do a search on HLLE here, by using the search feature at the top of the fourm, as well as by using your Internet browser. There is a wealth of information out there and you will need to become informed so that you can treat the fish on all fronts.
Take a look at the pictures in the Diseased Fish Thread located at the top of this form. You will see several fish there with this disease as well as a link to a thread here on a successful treatment for one of our members. This is treatable, and reversible.
Also, would you mind if I use your pictures to post in the Diseased Fish Thread? This will go towards helping other hobbyists id fish ailments.
Feel free to post back with questions.