What could it be?
They were covered with aphids most of the season despite pesticides. Then this unsightly stuff covered the leaves after flowering. The leaves seem to have discolored also, maybe because of this?
I found this info from Clemson, it goes into crape myrtle's major diseases (and their controls), which varieties are more resistant to bugs and fungus, and which to stay away from. I found out my Carolina Beauty is a bad one