It's hard to tell in thus pic if it's rotting. If you could take it out of the pot and then take a pic it would be easier to tell. The whitish stuff at the base isn't rot. What I can tell is that if this is the container your cactus has been in I think it's time to give it a new pot. I would get a terra cotta pot maybe an inch bigger than the container in the pic. Then get some cactus mix preferably without peat. Also get some perlite or pumice to mix in the cactus soil. A 50/50 ratio should be fine. Plant your cactus higher up in the pot:)

Soil looks dark and damp. Needs to be repotted in better soil. I agree with Kara, your cactus would love a terra cotta pot but not much bigger. What concerns me is the double pots. Is there a chance when you water it that it sits in water in the outer pot? (That should never happen.)

@socks I just watered today so the soil is damp now but I really only water maybe once a week only if the soil seems dry. There's no water at the bottom I usually have the plastic pot in a larger pot to hide it but this one just didn't fit. I'm going to try and repot it this week. Hopefully that helps.

Ok. Be sure to get better soil. Remember it needs all the light you can give it.