How do I remove the dark film that covers my crape myrtle leaves its a

Here is a good link:

https://homeguides.sfgate.com/black-residue-crape-myrtle-trees-26967.html

It is mold that grows on the sticky sap created by pests eating on the trees. Use a neem oil mixture and spray them, or dish detergent and water, then rinse.

Take a sample to your local Agriculture agent for advice.

For any plant or shrub that has either a fungus or insect problem, I buy Triple Action Neem Oil by Southern AG at Home Depot for $8.99. It is a concentrate so I use my tank sprayer to apply it. My roses, apple trees, rudbeckia, are now beautiful all season. If it rains, I reapply it. Make sure you cover the plants with the spray. I also had something going on with one of my boxwoods, so I used it on it as well. Problem solved.