Thanks for the quick responses! :) I haven't been able to spot any critters on the leaves(? fronds? branches? not so sure about terminology here!) so far. The cactus pictured is an Epiphyllum - I just did some research and identified it. Whatever is going on with it is also affecting my Euphorbia milii - there are little brown spots and holes appearing on some of the lower leaves. I water them sparingly with room temperature water and allow it to run through. They're located close to a window in my kitchen... I don't think it's too drafty there but I could be wrong. There are more pics of the problem in the link. Thank you for responding!

Probably a good idea to dig it out and separate it from the bunch. I've only dealt with mealies and these definitely look like something else. Hope you can find a cure for it, looks like a nice plant. Is it Crassula "Morgan's Beauty"? Could be "Coralita" as well

Mechanical damage and potentially slight over watering? The depression could be fungal.

I don't believe it is oedema. I don't think it is sunburn either - but possibly. At least on first plant. Ppl posted photos of similar damage, and it is likely caused by something chewing on it while still younger. Those 'bites' have scabbed over. As the plant grows, rosette opens up more and damage is more visible. You can see it is sort-of regular. Very young growth is usually most juicy/succulent, and it is often first to be chewed on. One doesn't have to see the bug or pest for it to happen...and just because other succulents were not damaged, it doesn't mean it didn't happen to this one. I actually witnessed this to happen. As Dominyka said, those leaves will eventually grow out, and plant will look better. April If you don't mind here is a suggestion: hope those metal tins have drainage hole(s). If not, they should be drilled or poked in. And if possible, top up the potting mix so the plants are not so deep inside of them. Metal heats up more than terracotta (didn't compare it to plastic); and plants are even 'hotter' if they are deeper inside. And it restricts air circulation too.