I don’t remember seeing anything like this before. Maybe I never noticed. But once I noticed these strange patterns, mysterious and fascinating, I saw quite a few examples.

We usually call these poplar trees, but they are also known as trembling aspen, or quaking aspen, for the way their leaves tremble in even the slightest breeze.

My best guess, from what seems like hours of online searching, is Epidermal Bark-Mining Fly. I can find next to no information on this.

Or could it be a fungus?