Looks like it could be leafminer damage.

Not leafminers. Instead, from chewers -- likely small caterpillars -- which are long gone. They scraped the surface off the leaf rather than eat all the way through the leaf.

How do you treat this?

There's little point in treating. The culprits are long gone and the leaves will be too come autumn. You can't reverse the damage and it won't hurt the plant.

i like to think.. that over 90% of the leaf is intact.. doing its work ... so there is nothing to worry about ...

ken