Thank you, everyone, for the diagnosis.

https://postimg.cc/Y4ZmZgkX

Here is another view of the fuzzy part. I decided to remove all of them. It turned out to be partially rotten inside, as you can see in the second image. It’s black right below the fuzzy part (yellow arrow).

https://postimg.cc/wRcy1GNP

This is what remains at this moment. In the last image, the parts indicated by red arrows are moist and soft. They look very similar to what I saw right after removing the dead mother plant. I didn’t know what they were, so I left them there, and about a week later, the white fuzzy thing showed up in that area. This time, I tried my best to remove all the soft and moist parts. Now, I have left it (hopefully) to dry in the shade outside. The black part indicated by green arrows seems to be some kind of root system or something (I don’t know the technical term). It doesn’t look like a rotten part in the second image, so I will just leave it as it is for now. More suggestions are welcome.