Gymnocalycium mihanovichii is a species of cactus from South America. The most popular cultivars are varied mutants which completely lack chlorophyll, exposing the red, orange, or yellow pigmentation. The plant is called a "Moon Cactus".
The ruby ball cactus, also known as the red cap cactus, is a grafted specimen. The colorful red top is a gymnocalycium, which refers to the flower buds bearing no hair or spines. While the lower green cactus could be any number of varieties (usually a Hylocereus cactus). The main job of the lower cactus is to display the gymnocalycium at an advantageous height. The ruby ball is an albino plant, which means it has no chlorophyll. Therefore, it relies on the rootstock cactus as a food source. It has a parasitic relationship. If there is a disconnect between the requirements of cactus on the bottom and the scion on top, one or both may die.