I have this beautiful 20-year-old cactus that was gifted to me a few years ago. It's starting to turn greyish in color, and those parts of the plant are rock hard now. Can I save it from dying? The grey parts have almost reached halfway up the 6-foot cactus. It's still healthy on the green on the top half. Please any help/advice would be much appreciated... This cactus is one of my favorite house plants.