I recently bought a Mammillopisis senilis cactus. Only half of the cactus was visible over the top of the pot, and the other half was hidden down inside the pot. When I took it out, I noticed the bottom half of the cactus was yellow. It isn't soft like rot, so I don't think it is rot. Is the bottom half just yellow from the lack of light? If so, will it turn green again? Any other suggestions?