heres another pic

Small, handsome globular cactus with both black and white spines need name. Thanks! https://www.flickr.com/photos/seedmoney/sets/72157651890301093

I believe it's a Echinocactus Grunsonii - Golden Barrel Cactus http://pics.davesgarden.com/pics/2005/02/07/cacti_lover/3fc57a.jpg

You've got a Euphorbia most likely Euphorbia trigona. Looks like it's beginning to rot or has fungus. The fact that you say it's spreading is not good. It will just spread all the way up. You probably will have to perform surgery. When you do cut it. Just use a clean sharp knife. It will ooze a white substance. Reminds me of watered down Elmer's glue, but this ooze is toxic. So wear gloves and protect your eyes. Everybody responds differently, but I don't think you want to test it;). You will cut the roots off, but that's ok. Cut until you see clean flesh. The bad parts remind me of bad melon. It will be brown. There should be no brown spots on your cutting. Once cut and all the bad parts are gone you can put the cutting under a faucet with slightly warm water. This will stop the ooze. Just get a paper towel or something to lay the cutting down on, so it can form a callus over the cut. This could take a couple weeks. Once a callus has formed you may replant not quite an inch deep. You will need to stake it up till roots form. Keep in a shady area away from direct sun. No water till roots form either. Euphorbias can take months to form roots. So just put somewhere where it's protected from the cold, water, and sun, and forget about it. I had to reroot a Euphorbia this year. That plant took about 5 months to reroot. So don't go checking every week because the roots won't be there. You also disturb the rooting process by checking on it all the time. Just be patient. Your mix should be fast draining. If you haven't, add 60% perlite into your cactus mix. This will help with drainage and aerates the soil. I know this sounds like a lot, but it's the only option when you have rot unfortunately. Well there is another option. Just buy another;). Goodluck:).