If it really is whitespot (pics would help), it can certainly kill lots of your fish quickly. I had a whitespot outbreak in my tank and lost half my stock within a matter days, even with a QT tank. Some fish never showed signs or symptoms of it, but of those that did, only 1 survived. There are however some other causes for "a spot" on a fish that are not so troublesome.
As for "reef-safe" treatments, I've yet to see the one that is both reef-safe and/or kills whitespot... All a bunch of snake oils that don't list why/how they actually fight whitespot. QT and hyposaline or copper treatment are still the only tried and true methods. Also to clarify, when treating with copper, you buy a copper med along with a test kit and add it until you get to a tested value of 0.2-0.3ppm. Do not have rock/sand in the QT tank if treating with copper as they will suck up the copper and make it difficult to reach a constant level of copper in the water column