I live in Hungary, if the climate is relevant.
The picture is too blurry for an exact determination. From what I see it might be Metcalfa pruinosa or a related species, which is an insect of the order of the Hemipteri. It's a widespread parasite of ornamental plants. Here a page about it
if true, what you posted are the nymph, and you might be able to find the adults around
This sort of thing happened to one of my plants. When I took a closer look at the plant, it was a group of white aphids. The brown coloration of the leaves of your cherry tree was relevant because aphids can sometimes kill the entire plant that they feed off of.
I did not take a picture of the plant, but I found one online: https://entnemdept.ifas.ufl.edu/creatures/trees/shivaphis_celti03.jpg
Woolly Aphids. They suck the juices out of the leaves of certain trees such as cherry trees and rose plants —to name a few.
Best way to treat them, is gather a plethora of Lady Bugs (or maybe two) to feed off these parasites naturally. Alternatively, try using Neem Oil. It is derived from the leaves of an Indian tree with amazing properties —so your trees/plants won’t be affected. You may need a spray bottle. Just add a little water to dilute for the spray. Make sure you get under the leaves as well. Best wishes!