Very hard to see from the pic but i've never seen a Naso Tang with anything like that on them.You didn't give any info. on water pars.How long you had the fish and what do you feed it.Naso Tangs are very hardy, keep your PH at 8.2-8.4 water should be very clean and keep him well fed through out the day on Brown,Purple and a little Green dried Seaweed/Seaveggies.Whatever it is, he maybe able to fight it off on it's own with out Meds by building up it's immune system.
looks like hes been rubbing against the rocks.. i had a naso that had that same look, he didnt last long, but i noticed that he didnt fit into the spot in the rocks he kept trying to go thru and it tore him up, looked like that.. but hard to see in the pic..
Doesn't look like parasites to me, when naso gets stressed dark spots shows on their body, not sure if this is the case. A better pic would definitely help.
+1 MotorTrendz.. When Blonde Naso's are scared or stressed they will camoflauge themselves entirely with a darker coloration on there body with small white spots aswell. The white spots appear mostly on the top base of the fish right before the dorsal spine. Not small black dots that are on the face.. I have to agree with Motor, thoose appear to be scratches.