Yeah I mean ich. We call it whitespot here in AU.
Previously I haven't QT but I do now (since I have a QT available)... There was previously an outbreak introduced from one particular store (they're known for whitespot) which I do not purchase from anymore.
It's been 2.5 to 3 years since the whitespot has been present, during which there's been new rock, a move to a new house, other fish introduced, inverts and corals as well. Since the initial outbreak there hasn't been any disease in my tank, except for a cyno outbreak about 2 years ago.
I'd definately be QT any new fish that get added from now on, so that's a given.
Still, my concern is about the tang "developing" ich even if it doesn't have it. I've been told, though I don't know how it's possible, that blue tangs can "just get it" even if it doesn't exist in the tank in a symptom form. It's as though it's dormant inside the tank and just attacks blue tangs really easily. Not sure how much I believe of the LFS
to be honest.