This mask is defined through a polygon and a feather parameter.
To edit the points of the polygon, you need to use the Edit Mode of the Viewport:

The Edit Mode is relevant to the currently selected element in the Element Tree, so take care that the polygonal mask is currently selected. Now, press [CTRL] and click in the Viewport to create points:

When there are more than two points but the polygonal mask is not closed, it will display a 50% overlay on the whole image. To close the mask, [CTRL] + [CLICK] on the first point:

The points of the polygon can be moved at anytime by dragging them in the Viewport. The only necessary thing is to be in Edit Mode with the polygonal mask currently selected.
To animate a polygonal mask, just drag and drop the "polygonal mask points" parameter inside of the Timeline. Now each key you'll add on this parameters will concern every points of the mask :


You could also animate all the mask by using Exposed Parameter on the "Points" parameters.