Whiteflies, probably.

Now, I can see wings on whiteflies. Those are small, but macroscopic -- a few millimeters across. I can't make out anything on these guys. We're talking something like half a millimeter. They look like white dots. But immature whiteflies maybe? Whiteflies do like brassicas. If really whiteflies, insecticidal soap should help.

Flying up in clouds sounds like whitefly. I ignore them and just wash them off in the kitchen. Good information here. Note that the link says complete eradication is neither easy nor necessary.

OK, I got out a magnifying glass and, best as I can tell, they look like pictures of whiteflies. So whiteflies it is. Thank you. But as I said, whiteflies are reported to be 1-2mm across. These are probably half that size or less. Here's a picture through the magnifying glass. Not quite in focus, but these guys are definitely elongated with wings. But yes, they're easy to wash off when you pick the leaves
for eating.

Interestingly, yesterday I sprayed with Bt salted with dishwashing
detergent. What I see now are mostly dead ones. I suspect that spray had
some effect. The dead ones I see aren't just whiteflies that have stopped moving. Kinda looks like they're falling apart. Does any one know exactly what soap does to whiteflies?