Video Editing "Alpha color channel" cannot be controlled by "RGB curves"

I am facing a problem. The problem is " I have rendered a shadow in cycle render with shadow catcher disabling the ‘Diffuse’ in ray visibility. The shadow catcher took not only the shadow but also noise around the shadow. Now I want to remove the noise and to keep only the shadow. The shadow color cannot be controlled by RGB color channel. In After Effect I can control with Alpha color channel. How can I control it in blender when I use video editing ?

Please Help me.