What could cause these spots on my Astrophytums?

[zagoric]

Thanks for any suggestions.

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[hegar]

The spots could be produced by either a fungus or a bacterium. If it is a fungus, you should be able to spot tiny round or slit-like fruit bodies or mycelial thread growth in the light-colored area of the lesion, if you use a magnifying glass or a hand lens. Also, there would be more lesions developing and the original one(s) would most likely enlarge.
Also, oftentimes you would be able to see a water-soaked margin and/or a yellowish halo at the lesion periphery.
I cannot see these things on your plants though. If there are no changes over time, you are OK.
Keep a close eye on these spots and if you do notice any change like those mentioned above, you may need to use a pesticide.