On first glance, looks like aphids or mites. The fact that they’re large enough to see at this distance on a camera makes me think they aren’t aphids, but I’m not super familiar with the species in your area.
For aphids, the best natural solution is a healthy population of ladybugs / asian lady beetles, their food source are aphids. If you saw less of them when you saw a lot of ladybugs, then I think this is the answer. You can find some places that will sell you ladybugs in different stages of life that you can then let loose in a greenhouse. Otherwise, you could try Neem Oil, that does the trick if you’re persistent enough in application, because aphids can be sneaky and hide on the underside of the leaves. Soak the plants!
If it’s mites, though, depends on the type. Those can be much harder to deal with with, and are even sneakier / survive through many rounds of treatment. Immune to some treatments, even. I think ladybugs may still help to some degree, but Praying Mantis will also do some damage in this situation. I had 3 Mantis in my garden for a while, the three Musketeers!
I don’t use chemicals, so I don’t have any suggestions on stronger stuff for either case.
I know it’s weird to think about, but you can buy them online, too. Just make absolutely sure you are home when they’re delivered! I think mantis may help with aphids, if you can find any of them hanging around (probably not).
I would suggest Neem Oil, then. You dilute it in water, and apply in the twilight hours. It eats away at the exoskeletons of the aphids, but it can also eat away at good insect exoskeletons, too. This is why you need to apply only once a day at a particular time, when the good insects are not active. You could just go crazy and kill everything for a fresh start, too. Aphids and Mites will lay dormant and keep coming back, especially in enclosed greenhouse spaces with lots of protection from the elements and outside bugs.