The photos are a little blurry, but it appears as if it could possibly be spider mite damage. The cottony white stuff is actually not any "stuff" at all, but more likely a damage pattern caused by the feeding mites, which eat away the juicy parts of the leaf and leaf tissue behind that is grayish. If you look closely at a damaged leaf, you should be able to see if this is the case or not. Spider mites are most active in hot weather and on tender new leaves.

Spider mites are very difficult to treat, so treating with a miticide won't really help much. And with trees as resilient as our native cedar elms, I would recommend just letting the problem run its course. The tree will most likely drop all of the damaged leaves and be dormant for a short period, but should recover and begin to put on new leaves fairly quickly. I would tell you to water the tree well, but we've had plenty of rain, so you shouldn't need to.