blotches on tang?

[ganowicki]

I have a powder blue tang, it has been doing well for a while, but this morning I noticed that he has whiteish blotches on his body. his activity has not changed and he is still eating. Is this something that I should be worried about and if so what is is and how do I fix it?

PS it is not ich, for those who have seen rabbitfish camoflage, it looks like that.

thanks

Gord