Can't really see it in the video, but when I hear "White powdery splotches" 2 things come to mind. The first and most concerning is Velvet. If it is velvet, you are likely to lose most (if not all of your fish). Velvet is a very fast multiplier and will quickly cover a fish in a powdery white coating, almost like powdered sugar. Most fish infected with Velvet will perish in 24 hours or less.
The other thing I think of is a bacterial infection, this can be more localized and can appear as a white patch/es on the fish. While this is not typically deadly, if treated, it is important to treat it properly. I would suggest removing the fish from the tank and medicating it in a QT.
Better pictures will help positively identify the issue. Hopefully someone else will chime in as well.