Motion Tweening a Mask

Then, between the first and second keyframe, right-click and select Create Motion Tween (or use the dropdown in the Properties panel) to tween the frames between the starting and ending positions of the motion tween. You can scroll the timeline to watch it move, just like an ordinary movie clip; in truth animating a mask layer is no different from animating a normal layer, because the same rules apply to the shapes used regardless of how Flash is told to interpret their display.

View my example of a motion tweened mask.