UPDATE
I first tried taking it out of the soil and most of the spongy parts fell away, leaving a white, thick, strong root/spine that's coming from it's middle and stems out into lots of white roots that go through the whole soil. Do you think maybe if I just cut off the rest of the yellow parts and leave the root it'll be fine? Or do I have to cut it too? Should I post another photo how it looks now?
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Is this cactus root rot? (see image attached).

I've noticed yesterday that the whole base of the cactus is all yellow/brownish, mushy (soft and spongy to touch) and the cactus is wibbly wobbly because of it when you touch it, threatening that it will fall over.

I may have overwatered it even though I've been making sure it doesn't get much water and now the soil is mostly dry, so perhaps it had been overwatered few days ago. It's on a shelf that gets lots of sunlight from the window next to it.

What should I do? Should I wait if it dries? I think the spongy part is spreading though. Or should I just cut the yellow, squishy part and put it in a dark place and wait for it to regrow roots and then plant it into a well-watered soil and wait for the soil to dry? (as I have read on these forums and elsewhere)

Thank you.

This post was edited by Cacir on Fri, Apr 12, 13 at 13:23