If you have overwatered your Lithops, the first thing you need to do is stop watering it. Allow the plant to dry out completely before giving it any more water. Once the plant has dried out, you can start watering it again, but be sure to water it less frequently than you did before. If your Lithop is still not looking healthy, you may need to repot it in a fresh, well-draining potting mix. It will fix the issue of Overwatered Lithops.