If it is actually green moss growing on the trunk, it is not harmful to your trees, and it is not only a crape myrtle issue. Moss will grow on the north side of many trees in shade and moist conditions, which is not ideal conditions for crape myrtle.
If you are talking about a white powdery substance on the leaves, that is powdery mildew, which won't kill the tree, but will look bad and ruin the flowers. You can search powdery mildew for options for control of that.

Also ball moss will grow on dead branches in my part of the country. It won't harm the tree either. If you want you can just prune out the dead limbs that are hosting it.

Here is a link that might be useful: Ball moss. Scroll down a bit for a picture ....

Thank you both very much.
Janet