Gymnocalycium Cactus, also known as the Ruby Ball Cactus or Red Ball Cactus, are actually a grafted specimen. From SFGate.com – The Gymnocalycium cultivar — sometimes called ruby ball cactus or moon cactus — is actually two cacti in one. A pure red cactus seedling lacks the ability to produce chlorophyll and will die unless it is grafted onto a green one. The green feeds its mutant mate sugar molecules produced from water and carbon dioxide. Once established, the two parts grow together so you can’t even see the seam. These cacti typically live only a few years and do not grow appreciably, making them ideal for apartment dwellers.
Light: The Gymocalycium tops are tolerant of more shade than many cacti and dislike direct sunlight. By contrast, the stock green cacti on the bottom are often light-lovers. Look for a bright area, but not so bright that the cap begins to wash out.
Water: Allow the soil mix to become nearly dry between waterings, but then water thoroughly. Immaculate drainage is essential, so never let the pots sit in water. Suspend watering in the winter, but mist occasionally.
Soil: A rich, fast-draining cactus mix is ideal.
Fertilizer: During the growing season, fertilize with a cacti fertilizer mix. Suspend feeding during the dormant winter period.