They look like white aphids to me.
I see lots of them in parks, in shaded areas. They tend to make a fluffy white web.
I’ve been recommended a traditional remedy, however I haven’t tried it myself yet:
make tea out of these leaves (en.wikipedia.org/wiki/Artemisia_absinthium)
after it cools down completely, spray it over the leaves.
In Eastern Europe, this tea is common in supermarkets.
Yes, I would say whitefly as well. Increase humidity. Neem oil works but slowly. Using a loupe and identify eggs and caterpillars, and remove with a cotton stick dipped in rubbing alcohol is very efficient but time consuming. A yellow sticky trap prevents the adult flies to lay new eggs.