From my knowledge of yellow tangs this is normal(even though i havent the slightest clue of what it is). All yellow tangs have them it is just some are more viable than others. I knowticed that too when my step bro bought one and didnt think i saw it at the store but sure enough when i returned to the store and took a closer look i could see it on all of them but some better than others. Maybe it being more visable on some may indicate something but I not sure what. I saw if it looks health yand is eating and acting normal than it is a healthy fish. I sure hope someone else can actually tell you exactly what it is though.