here's the photo

If it’s mushy then yes it is overwatered. You can cut the rot off and let the other half heal. Don’t water it again until spring. Your cacti really don’t need water in winter. Unless they’re shriveling from dehydration, I would stop watering altogether for now. Especially if they’re indoors. I became quite ill last year and didn’t water my outdoor cacti for over a month in triple digit heat. They survived just fine. So don’t be afraid to underwater.

thank you very much