An ailing cactus

An ailing cactus

I suppose there are posts like this every once and a while. I have a cactus with a problem and unfortunately there's very little I know about cacti or taking care of them :/ This one has been with my boyfriend for about 7 years, I think, and seeing how long it has survived I'd hate to see it die on us. I don't even know what it's called. It has developed a spreading yellowish area (starting from the base) and I'm quite clueless as to what might be the cause and whether it could be cured. Various sources recommend cutting of the tip when the cactus is rotting from the base, but the thing is, I'm not sure if it's actually rotting. Plus the roots seemed to be fine (at least they didn't seem to be rotting to me) when I attempted to return it to it's spirits by re-potting just recently. But of course I could be mistaken, since I don't really know what cactus' roots should look like - healthy or rotting. So, the long story short - I tried my best to get good enough shots of the problem and am hoping that people with more experience might be able to help me on this. Thanks in advance to everyone! Apologizes to anyone seeing this posted more than once.
Japanese Bedroom

Japanese Bedroom

What kind of vine is this?

What kind of vine is this?

Sparaxis – harlequin flower

Sparaxis – harlequin flower

Productive weekend

Productive weekend

Slugs? Or… something with teeth? Help me out here.

Slugs? Or… something with teeth? Help me out here.

The cactus that would (NOT) be dinner..and peas!

The cactus that would (NOT) be dinner..and peas!