It looks like your Consolea rubescens has a fungal infection - that's often caused by soil that doesn't drain well or lack of air movement. What's the soil like? I'd depot it and check the roots too.

Its potted in cactus soil, with stones at the bottom to allow drainage. I try not to water more than once a month, and not at all over winter, after a different cactus rotted from the inside out :(

Lack of air movement could be the issue though. Will check the roots and repot in fresh cactus soil.

Get rid of the rocks - just use a piece of screen / newspaper to cover the drainage hole. Watering more than once a month is usually needed during its growth time. I'd guess the soil you're using is not porous enough (most commercially-produced cactus soils don't allow for enough drainage - they're great for jungle cactus types but not the types that need better drainage).

Thanks for that advice.

Is there anything I can do to make the soil more porous (anything I can mix in)?

What is that yellow 'bump' in your last pic above? I can't tell...

Yes, you can make your mix uch better by adding lots of pumice, or sifted perlite. PPl using that mix do it most often in 1:1 ratio. You could also add grit of about same size as pumice/per;ite, and mix them approx. 1:1:1. Or you can skip the soil altogether - but many are afraid of not using any. I don't use soil, just seldom, and if I do it is usually no more than 10-20% of total.

Not really sure what the yellow blobs are, there's a few of them on the plant. They look to be empty.

Thank you for the pumice advice, does this apply to other cactus species as well?

I use perlite since pumice is difficult to find around here. I also use grit, and mix them as described in my previous post (seldom I add some small amount of soil) - for all my succulents (approx. 300 pots..)

That yellow blob looks like a bug to me, maybe just leftover exoskeleton from it. Do you or did you have some scale or similar bugs on it?

Ahh thanks, I shall go and get some perlite/pumice/grit.

I had the same thought about the yellow blob, but haven't seen any bugs on it. The yellow blobs appeared at the same time as the grey patches.

Rebecca, you don't say where you live, but....if it's in the lower 48, you may have a Tractor Supply store near you. I buy Manna Pro chick grit there....sort of large bags (25 lbs I think) if you grow smaller plants, but....worth it to buy once and store. I have found this to be the most evenly sized small chick grit. I don't sift it --just wash it outside with an old kitchen colander.

Like Rina advises, I use A LOT of pumice, and a little perlite too. When I want to add something gravel-y to the mix that has a little weight and holds NO WATER, I add Manna Pro. I only have 5 or 6 real cactus plants but they are in soil, pumice and a healthy handful of Manna Pro.

Thanks all for advice.

Have just taken it from old pot, the soil was very compacted around the roots so gently took that off and repotted in a mix of cactus soil and perlite with more perlite than soil.

The grey patches are rock hard despite looking furry and soft, so wondering now if scale more likely than fungus?

The yellow blobs are also hard but are over where the spines are.