Depends on how picky you are but I'ld cut off the messy arm.Had a fine specimen that was doing very well, huge and blooming but fell over and cracked it's pot and there was no dirt there just roots, so I put it into a bit larger pot and it is doing better than ever. Sometimes the scabbed branch looks messy and might rebuild a couple of new shoots but will always look non professional and eventhough I'm just a homemaker after all the work and care I like my plants to look their very best. That's what they do at the nursery so we think we are buying some healthy plant but in truth they just chopped off the dead portion.

Now it's Bald Old Man of the Andes... You didn't say what precipitated this condition...Any change in growing conditions? A little background might be useful. Christopher

Hollow is no good anytime. OTOH, all of us who have grown these plants for a while have had something similar, if indeed not the same thing, happen to us. Photos will help, but if it's hollow, it's likely a goner.

I would agree, the second plant definitely looks like scale or mealybugs, and a pretty severe infestation as well. I would spray off (maybe using a soft toothbrush as well) and then spray with a mixture of water and 70% isopropyl alcohol (rubbing alcohol), either 2:1 or 4:1 you want to be cautious, and a few drops of dish soap or insecticidal soap. Some people will use neem oil or insecticidal soap, but I think that stuff is messy and smelly and I don’t like using it indoors. You also have to keep plants out of direct sun for a while if you use anything with oil because they can burn. Try and remove all the scale if possible. They shpuld be pretty easy to scrape off or brush off with a little effort, mainly around the spines. keep the cactus away from your other plants because they can spread. If youre not super attached to it it might be worth tossing, but if You’re diligent and repeat this every week for a month or so you can probably save it thought it will need some recovery time. Good luck!