I’m still puzzling over this. Has anyone seen anything like this before? It’s certainly distinctive, but I can’t come up with any theories for what could have done this. It’s on the underside of a live leaf.
Updated: I’m adding a crop from the above photo (from the upper right, rotated a bit).
Maybe this will help. There’s a tiny tiny critter about one third of the way from the left, near the bottom. I suspect it’s unrelated, but maybe not. I’ve also been thinking that maybe the clumps perpendicular to the main leaf vein might be concealing some sort of hemipteran nymphs. But then what about the oval perimeter? Maybe that serves to discourage predators, like a sort of fence?