Your cactus looks great:)! Looks like it's beginning to get what's called corking. Keep your eye on it of course because if the corking triples in size in a month there is something else going on, but from the looks of it now your cactus is starting to show some age. Just like we get wrinkles when we get older, cactus get corking:). Corking also helps with the strength of the plant as it grows:).

I would try to get some type of gritty ingredients, such as Pumice, Chicken Grit, Turface, Perlite, etc. Take the plant out of it's pot, and wash off all the old, compacted soil away from the roots. Don't be afraid of damaging roots. Wash it all off. Then, I would plant it in a mix of 50%grit/50% soil. You will have to place a stake or stones to support the plant until the roots have had a chance to colonize the pot and anchor it. If you can, place it in a warm, bright spot for a few weeks. When the plant is more established, start introducing it to Sun again. Maybe 1/2 an hour the first couple of days, and then 1 hour a day. Gradually increase the time.

I am sorry, but that plant is toast. The stock, the green bottom part, has rotted away, it is dead. The top is a form of a Gymnocalycium cactus that cannot produce chlorophyll. It needs to be grafted on another cactus. Theoretically you could regraft it, but the top looks bad too, and it is simply not worth it. Buy another one, plant it in a pot with holes in the bottom, and use half cactus soil with half perlite. But these cacti never grow really old. Try a regular cactus instead?

Deb, Opuntia are a different kettle of fish than the rest of the cactus family - they'll grow under difficult conditions much more easily than their cereoid cousins and you can get away with a lot more ill-advised culture with them and not have a problem. I think what you have is an Opuntia that's in the Consolea subtribe. The more room you give it, the more it will grow, so it's a good idea to keep it contained in the pot that you have it in.