If your living stones, or lithops, are turning brown then this might be edema. Edema is when the plant’s root system has absorbed more water than it can handle. The brown spots could also be caused by root rot. Root rot is when a plant is overwatered and the roots begin to brown and get mushy.
If it is a mild case of overwatering, you can leave the living stones to completely dry out before you water them again. You will know when to water the living stones again when they begin to look shriveled or are drooping down.
If it is a more severe case of overwatering, then you will need to inspect the roots. Remove the rotting roots with clean, sharp scissors, disinfect the pot, and repot the lithops with new soil.
When in doubt, don’t water. It is easier to hurt the living stones by overwatering than underwatering as they are used to hot and dry climates.