Why are some of my succulent leaves turning black?
It sounds like this plant has been overwatered.. easy to do with succulents so don't feel too bad. Succulent plants do not like to sit in water and will rot if they have had too much. To check, push back the moss and check how moist the soil is. You could possibly remove the plant from its pot and let it dry out a little but it may be beyond help if it has started to rot.