A moon cactus usually combines a colorful top with a separate green rootstock, so care has to suit both parts of the graft. The join should stay firm and dry, and the pot must allow excess water to escape.
Two plants joined together
The familiar bright red or yellow moon cactus is commonly a colored cactus grafted onto a green rootstock. The colored top is part of the living plant, not a permanent flower. Look for the join between the two sections when checking its condition.
Keep the roots draining freely
Choose a pot with drainage and a suitable cactus growing medium. Check the compost before watering, and allow surplus water to leave the container. Do not keep the rootstock standing in water or repeatedly wet the graft as a substitute for watering the roots.
Watch both parts of the graft
Introduce changes in light gradually and protect the plant from damaging cold. A firm top does not prove that the base is healthy, so inspect both sections for softening or collapse. If the join fails, this is a grafting problem rather than a reason to bury the colored top and treat it as an ordinary cutting. Ask a cactus specialist about the options for the identified plant.
Related reading: Ariocarpus Types: Comparing Rock Cactus Shapes.