If it is the mirror head or stem mount that is moving they can be tightened. If the mirror itself is movong inside the outer shell it is likely a loosening or incorrect manufacture process with the locating / fixing points of the mirror.
@ozrider had one go like that and was able to prise to carefully get a thin blade under the mirror edge and gently prise the mirror off.
He found they were glued at 3 or 4 points with a dob of standard yellow hot-gun glue to the mounts inside and one of the mounts had broken or been incorrectly attached at factory and detached. He fixed and then re-glued the mirror on and it works fine I believe.
To remove the mirror itself pry/lever GENTLY with a thin blade but this will be MUCH easier with GENTLE heat from a hairdryer on the mirror glass first to soften the hot glue.