My stem is molding is there any way to cut above the rot and plant the top into soil? If so what type of soil?
You can use a sharp knife to make a clean cut above the rotted section of the Coral Cactus. Let the cutting dry in the open air for about 24 hours. Use a cactus potting mix that is intended exclusively for Cacti. Do NOT use a Miracle-Gro mix that says it can be used for Cacti and Palms and Citrus. If you cannot find a good Cactus-only potting mix, then mix two parts of peat moss with one part perlite to make your own porous potting mix. Use a terra cotta pot that is 6-inches or less in diameter. Insert the cutting about an inch deep into the pot. Provide lots of warmth and sunlight for best results.