FINAL CUT PRO ... grey edge on black 'Draw Mask'???

I have a patterned detail in a shot that is vibrating. I just want to mask out the vibrating/shimmering section. Which should be easy. But my 'Draw Mask' is adding a thin grey line to what should just be the black mask. See attached shots. It's simply a chair with a webbed back that is distracting the eye. The shot is moving a bit. I've used keyframes. But the grey line moving is now distracting the eye. I tried a Flicker filter but couldn't get it to work. Any ideas?