Cactus experts assemble, please

[CarterBeatsTheDevil]

Any idea what this grey scab/cork on my cactus is? It's hard and dry, not squishy, and doesn't rub off.

Corking at the base of cacti is usually quite normal - due to ageing. But if this is something new, that would be more worrying, and might be a sign of root rot. Your photo is not very clear, a shot of the whole plant would help.
You could unpot it and inspect the roots for signs of rot, and it needs to be in a well-drained potting mix.

Thank you! Hopefully clearer photos attached. It's in cactus soil with gravel at the bottom and perlite mixed in.

@NanTheWiser it's still very firmly rooted - no give on that side.

It looks quite healthy, probably an Echinopsis, so maybe just old age. It could do with being planted a bit higher in the pot, but otherwise I would leave it alone.

Yes, I've been toying with the idea of repotting it now, actually - not sure what I was thinking of. I wondered if it was just paler green and less full down there because of lack of light.