I think I saw that a while ago on.....ReefBuilders? I'm pretty sure the guy's not for sale :)

If you look in David saxby's tank he has a whole bunch of piebald (Yellow, black and white) yellow tangs - but none of them are stunning like Casper!

Does anyone know if that's actually a separate species, or is it just a super rare colour variant of the yellow?

It's "just" a variant of yellow. If it was a separate species, we'd see a lot more of them :)