Green patina on copper or alloys that contain copper can be forced with weak acids like vinegar. All human sweat is acidic, some more than others. That may be a part of what happened there.
If you feel comfortable with taking the covers off and replacing them, you can remove that green patina from those areas with gentle brushing and then soak the covers in liver of sulfur to force a black patina, but I would check their development often to avoid over coloration and definitely follow up with an aerosol sealer to prevent it happening again.
