After putting on a halloween mask and looking at their reflection in a mirror, young preschoolers often are wary. this is because they do not yet fully grasp

True after putting on the mask it does spook the kid and makes him a bit wary only because he is caught off guard. If the kid gets acquainted to the situation and more "used to" wearing the mask this fear my go away. I think the kid will have fun and not worry after getting used to the mask.