Okay. From your description of turning the screw both ways does not loosen the grips, the escusteon is loose in the rubber molding.
You don't have much of a choice now except to somehow immobilize the escusteon to remove the screw, or forcibly open the grip halves, and pop the escusteon out of the rubber molding.
I would attempt to immobilize the escusteon first. Try prying the grip halves apart at the seam, and forcing the half away from the screw as you unscrew it. That might be enough to stop the escusteon from turning.
A second, less desirable approach, is to cut with an exacto knife through the rubber to access the escusteon, and use superglue like I described ealier. Or drill out the rubber to get to the escusteon. Either of these is likely to leave visible scars on the rubber.