It's the root system but I don't understand how it could have rotted I almost never water it. What do I do with cinnamon powder? Should I let it dry before I cut it?

Let it dry and yes cinnamon powder before you do any thing also listen to other peoples advice

Hi, Celeste! I don't grow any cacti, so I would defer to those who do for any specific advice. However, that white stuff looks like fungus to me. It also looks like it may have some mealy bugs in it. SO - if it were my plant I would investigate what the white stuff is before doing any cutting.

Take a q-tip dipped in rubbing alcohol and rub some of it off, or even spray some alcohol on it. If some of the lumpy parts turn orange, it is probably mealy bugs. If it is a fungus, it still needs to be removed and dried out. I have never had root mealies (knock wood, fingers crossed, etc!), so I don't know if that is your problem.

Did your plant have a nice root system before? I know how disappointing it can be to suddenly have a problem like this. The rest of the plant still looks healthy, so there is indeed hope for saving your plant.

It's not mealies I had those before & the fuzz looks similar but I don't see any actual bugs. The root system was fine everything was going really well. If is a fungus what do I do? I used rubbing alcohol & water to get rid of most of the white stuff. It was also in a dish with 2 other cacti, are they in danger?

How far up do I cut????

Just curious on what the tissue looks like where you were able to wash the white fluff off? I think it looks like root mealy myself, but if youre sure its fungus, you would cut until you have healthy green tissue, stearlizing the blade in alcohol between each cut. Then treat with cinnamon and leave out to callus over. But i wouldnt cut it unless it looks like it cant be saved otherwise. Whats it look.like now after the alcohol wash?

This is the bottom now. Do root mealies look different than the kind that attack the top of a plant? Because I can't find any bugs at all

Hmmm. Did you cut already? Or was this what happened after cleaning? Maybe i should refraise that, is it hollowing out on its own? Ill be honest, If it was mine, i think id cut it until there was healthy tissue, past the pitted part. But if you havent cut anything, ill be honest and say that Ive NEVER seen something pit out like that that quickly without being rot but I dont see rot. When i had root mealie I never saw the bugs, just the white fluff on roots and it was an aloe which rotted VERY quickly. But the rot was blackish, the plant basically melted. I would honestly wait for someone more experienced with something like this. But if it does continue to eat away before you get a response, cut it until you see healthy tissue stearlizing the blade in alcohol between each cut. Im sorry I couldnt help more, in just not comfortable enough in giving a defenitive answer. Hopefully someone more experienced will come along and give you a definate answer.