My crepe myrtle has a lot of sticky sap and white fuzzy spots on it. It is about 12-16 feet tall with several large trunks.
The sticky sap is honeydew and is caused by insect pests, likely mealybugs, scale or aphids. Treating the tree with Neem oil spray should help. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm