Hi, 

It looks like it has powdery mildew which is a common symptom that can occur with beebalm. Despite the full sun, our humidity in the mid Atlantic region can cause this. To manage this its best to select resistant plant varieties but even those that do show resistance can still get it. Some gardeners can get control with horticultural oil spray but you need to check the label to see what temperatures it can be used in. 

It doesn't usually harm the plants but does cause it to not look as pretty. 

You could also try to 'Chelsea chop' some so they are different heights to give it some more air flow. You would cut some back to about half height earlier in the season and let some bloom first, then as the others bloom you can trim back the spent flowers of the stalks that bloomed first.