'Scale' on mammillaria?

[Russell99]

I does look like mite damage but at the same time it looks like scale also. Try to see if you can peel off that using a pin, tweezers or your finger nails

[Nory]

Thanks everyone for the replies - I've tried scraping some off and it doesn't come off too easily, but when it does it seems to be fine underneath, just has a 'fresh plant smell' for want of a better description... The tubercles are fairly soft though, and difficult to reach, so it's easy to scrape the good bits off underneath the 'scale' and I've already taken too much off in a couple of places!

There are some bits which look like dead flowers that I hadn't noticed before, but I've never seen it flower and though it was at my parents' house for a while, they don't remember it flowering either... so could these actually be some sort of mites?

Maybe I'll have to take it out of the pot and investigate further? A jewellers loupe is now on the shopping list!

[Ali Baba]

That could equally well be thrips damage. If it is mites you will be able to see them or their eggs through a decent x10 lens. If it is thrips you will find no sign of anything!