Is it age? Some of the stems and leaves look like they are dried up (but they aren’t) and some pads have a crust-like covering on them.
Some cacti, like holiday cactuses, can get woody bases as they age. But there could be bugs or fungus at work. You may want to propagate a new plant, just in case this one succumbs. These articles should help:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-scab-treatment.htm