definitely a mimic tang, not a yellow tang.
i mean, it is a tang that is yellow, but usually the common name Yellow Tang refers to Zebrasoma flavescens.
anyways.
that could be the start the of HLLE, or it could be stress coloration, or it could even be the start of the adult color transformation, since they appear to have a totally different color scheme when they reach adulthood.
what is your feeding regimen like for him? is it possible that he's not getting enough algae in his diet? alkalinity on an acceptable range as well?