Moon Cactus is a great cactus for beginners. It grows well indoors, and thrives on neglect.
It is actually two separate cacti grafted together. The top, colorful graft comes from Gymnocalycium, with the root-stock cactus coming from another cactus. The colorful Gymnocalycium is a mutant, unable to photosynthesize, and therefore requiring nutrients from another cactus.
2.5" size pot