The powder could be 1 of 3 things.
#1 Magnesium Carbonate residue. Some taxidermists use it to mix Peroxide with and than paint on the skull to whiten it.
#2 Residue from using 40 Vol Hair Developer with the bleaching powder.
#3 Could be Sal Soda/Powdered Laundry soap from when the taxidermist boiled/simmered your skull.
If it is #3 I doubt it will come off easily. If #1 or #2 it should come off fairly easy.
I'd take it back to the taxidermist and address the issue with him. Since I can't see the skull I can't do anything but speculate as to what it is.