Um, what kind of trees? We have oak blooms that look sort of like that. It piles up and I just use it as mulch. Though it does tend to blow around.

That is actually flowers. I have both water oak and pecan trees that make these. In fact the pecan trees are covered right now. They won't self plant, but they do make a mess. Catkins are what they're called.

Answered on duplicate thread.

Catkins! Right.

And just to clarify further, catkins are the little boy flowers, the pollen carrying structures. Pollen grains carry the sperm cells. Thus, no worries about using catkins as mulch or leaving them where they fall.