I have that on my pumpkin patch, I have 3 vines and only one has that white film like residue, however it only occurs after it passes a piece of wood (which does have fungi on it). This doesn't answer your question, but I want the answer too.

Many varieties of squash have a natural silver mottling. It can be quite attractive.

Both my Giant Argonaut butternuts and my Wautaga Pie Pumpkins have that patterning. Its common among zucchini varieties as well.

But it's only on this one leaf? Is that normal too?

I don't know why either butI have seen it too. Usually, very healthy vines do have some leaves like that. Eventually it should disapear. I woul not worry about it as long as it is not mildew and the leaves are healthy.

cyrus