Can you provide a picture of this 'mold'. Is it on the leaves or the bark? Could it be scale?

Yup, scale and the associated sooty mold.

Yes - sooty mold is what it looks like. How is that treated?

The sooty mold in itself is not harmful......unless it covers all the foliage. That can affect photosynthesis. But if it is just scattered around, you do not need to do anything to it, although it can be rubbed off with water.

To actually treat for sooty mold, you need to get rid of the source, which in this case is allegedly scale. There are a number of different kinds of scale and they are typically treated differently, although a systemic will work for most types.

Get rid of the scale and the sooty mold will disappear.

The white crape myrtle bark scale has moved up here to the mid-Atlantic as well. It's everywhere. I don't know the best remedy. You could google it. I've seen recommendations for using a soil drench of an imidacloprid product. You can also spray with a dormant oil when the trees are not in leaf, but of course that's a pain with tall trees.