Re-pot them into a new mix.
I grow several CC's in pure (or nearly pure) fir bark - sold as Orchid Bark, fine grade.

I couldn't even guess what the pest might be. Kill one, and then take a picture.
They're real slow once they're dead ;)

Josh

They might be a symptom of something else amiss, rather than the cause.

Al

cliss, you'll need to give us some more information so that we can better help you. Where are these plants located (outside, always inside, greenhouse, etc.)?

Can you come up with a good description of this critter? All you've given us thus far is " fast, gray" and that's not much to go by, lol. Size, shape, antennae, legs, etc.

I agree with Josh...try to 'get' one without smashing it too badly. It's kinda hard to identify really flat bugs.

I've already tossed the soil from the affected plants, but if I lose another, I'll try to grit my teeth long enough to get pictures. :) As it stands... Shiny gray, no sign of segmented shell that I could see (i.e. they didn't look like "roly-polies" though they were close to the same color), about 1/8 inch long and half as wide, fast-moving with a six-legged crawler gait (i.e., not like a grub, a gnat or the eight-legged mite clan). No antennae that I could see in the brief glimpse I had.

And I've realized that I miscounted -- I've only lost four cacti, I'm just expecting to lose a fifth for probably-unrelated reasons (roots badly damaged in transplanting and not re-growing well). Of the four I've lost, one was a plant that I've had for a couple of years in a humus-based potting mix (Hyponex), one was a recent acquisition in a commercial cactus mix, the other two were in modified 5-1-1 (ground fir bark in place of peat). The one with root problems at the moment is also in that same 5-1-1. It's certainly possible that there's something wrong with my watering method, or in the case of the younger ones that I screwed up something when transplanting...