Grafted cactus rot?

[allygracebz]

The bottom of the lower cactus (as seen in picture) has turned a significantly lighter green. It is not soft nor is it broken inside, but it is thinner in that portion than the meatier green parts above. I've encountered this appearance before, but I was broken and seedy and due to too much water, causing rot. After losing that cactus of he same kind, I have been very careful to not water this one very much, so what could be wrong?!