Looks like thrip damage to me. They'll make an opaque window out of your leaf. There's alot of it here at the moment outside in my garden - we're supposed to be having summer and the little beesties are about. If you have a scope or strong magnifying glass have a look at the damaged area under the leaf. You may see black or brown dots which are thrip poop.
If in doubt maybe post over in the Plant Problems.