Naso Tang disease? Please help identify.

[newreefer_59]

Not sure if the pic really shows it, but I have an Orangespine Unicornfish (Naso Tang) that has developed a greyish bump on the white band between the yellow-orange bands nrea his tail fin. There are also 2 similar but smaller ones on the tail fin itself on the other side. Any idea what this may be? I have had him for well over a year and still eats and looks otherwise completely fine. My tank is 150 gallon reef tank.

Thanks in advance.