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!

To make sure it is scale, try to wipe them off with a cotton ball/q-tip moistened with rubbing alcohol (or even a tissue). Scale would come off easily, with just little pressure. If squished, there would be red 'goo' - blood. Otherwise it is not scale. In last photo, the tips are white (not brown). Could be just drying of the leave or it may be mealies. Is there any fluff? cottony-looking very fine webbing? See if it could be wiped off. Are they all potted in communal pot? Some have different watering needs. Btw, 6feet away from window is typically not enough light for most succulents. Rina

They need lots of sunlight. Maybe still not enough indoors. Any obstructions (trees or other vegetation, curtains...) will make light 'indirect'. I think that spots may have started as mechanical injury; but check 2 bigger ones if the leaves or spots themselves are soft. If there is a scab over, it should be OK. Otherwise, maybe even remove those leaves. Make sure the potting mix is thoroughly dry before every watering; use bamboo/wood chopstick or skewer or even pencil to test deep inside the pot. Bottom plant doesn't look that great - hopefully only because the protective powdery coating (often referred to as 'farina') seems to be rubbed off. Try handling the leaves as little as possible. Check the leaves if they are soft/limp - they should be quite turgid. IMO, it needs more light. If you have access to any outdoor space, they would benefit being there - but do not put them into sunlight suddenly or they will burn. Outside light is more intense and plants should be acclimatized. Can't tell for sure how low is the soil level in that pot, I like plants sitting higher so no leaves are really inside of the pot, if possible.

Just thinking detective style. You showed us the gorgeous basket of healthy plants. Did you buy/obtain them and brought them home in the basket? Now all the plants that were in the basket (except the Jade) have this "malady". Then there are over 50! plants with the same malady. I'll assume a bit here until you correct me. You have a large collection of Succulents, so a pretty experienced succulent grower. All plants are ok. Then you bring home a basket of Succulents for a group planting. A week after replanting, the malady appeared on some plants in the planter, then on the others (except for the Jade). Within the next few days, over 50 of your entire collection has this malady. There has to be a connection... Same growing conditions causing malady to many plants? eg Oedema. Insect and/or lavae/eggs came in with basket? Something originating from the planter? eg something left from previous plant/s. Anyway if you're interested... could you rewrite the "happenings" correctly and with as much detail as possible. Eg I bought the new planter from bunnings or guy next door gave me the planter when he moved 3 weeks ago. First sign of malady- day/month time I noticed on #of plants in planter. If your not interested, that's cool. Also wondering if you’re into the fast draining gritty mix that rina was talking about. I use it now. Incredible difference in the plants health, not being stuck in unnatural cloggy dirt. BTW What general area in/around Sydney are you in. I spent a lot of time going to Richmond to stay with a family friend during her last years. She died, content, about 2 yrs ago. I would have been visiting about 3 years, several times a year. She was like everyone's favourite Aunty when I was a kid. Sorry that was just why I was interested lol