It's hard to say from the pics, they're clear, just obscured by the other plants...
The moon cactus LOOKS ok from here, what are you concerned about with it?
The old man cactus does not look good!
It might be curtains for that guy... Never too late to try tho!
To me the biggest thing to look for is rapid change, if that yellow is new, and moving fast... Not good!
Is the plant soft?
Or maybe oozing if you squeeze it!
Try to find a spot out of sight and grab a spine or 2 and pull hard, if it's rotten you'll know it!
If it were me, I'd promptly pull the plant out!
If it's root the root will likely be dead, if they are, then get a sharp knife!
Start cutting away dead rotten tissue, just keep working your way up until you get to healthy looking plant.
Again, it should be obvious, they tissue will be soft and dead, not crisp, you'll likely see brown spots...
Once you get down to good plant, then you need to let that cutting scab over, it'll get a hard brown skin over it, it'll take a few weeks, then place it on top of very sandy soil and do not water.
With luck it'll root up and grow.
You can read for days on this topic.
You also mention grey spots, I don't see any from here...
Look up mealy bug and see if that's what they are...
If your plant turns out to be ok, it'll still be good that you pulled it out, that type of cactus likes to be pretty dry at times, I can't imagine it being able to happily share a pot with regular leaf plants like it appears to be...
Good luck, sorry your plants not happy
