Hi Sonn!
Yes, you can use "flying" as a verb for the way poplar fluff moves, particularly in the wind.
Although we often use "flying" as a verb for the movement of animals with wings, or winged objects like aeroplanes, it can also be used to describe the movement of lightweight objects when they are falling or travelling.
However, this is usually when the objects are travelling fast.
If the poplar fluff is falling gently, say "drifting," but if it is moving quickly (maybe because it�s caught up in a gust of wild," then "flying" would be perfect.
Hope this helps!