Edges of effect show up in animation in Motion 5

I'm trying to animate a lens flare across the screen. When it moves across I can distincly make out the rectangle lines that makeup the perimeter of the effect move with it. It's as if you moved a pane of glass with a design in the middle in and out of frame. The edges of the glass show up. I must be doing something wrong. I would think the edges of a lens flare would taper out to nothing. How do I avoid seeing these sloppy edges? BTW, I'm rather new to Motion.