I think you cant get rid of it. I remember my grandma crepe myrtle use to have the same problems. She spray it with something from home depot. It got rid of it but it came back. The dark pink one is the only one that had that problem though. She had white, red, and the dark pink ones. The other ones were not infected. All 3 of the pink ones were infected though. I try to get rid of them by spraying water or by removing it by hand lol. I was a little kid then. IT was kind of sticky.

Looks like you've got white flies and fungus. I'd give those leaves a blast of Neem oil with a hose end sprayer. Yes, some crepe myrtle seem more susceptable than others, but it will recover.

This link suggests spraying with plain water and/or insecticidal soap.

Here is a link that might be useful: White flies and fungus on crepe myrtles ...