The Mask Task is related to the seasonal world event Hallow's End. If you destroy the mask you have received, the criteria stay completed. Note that masks are BoP and cannot be traded.
All the masks can be purchased from Spanky just outside Stormwind City and Farina in the Ruins of Lordaeron for 2 . Some of them can be found within [Handful of Treats] and the [Loot-Filled Pumpkin].