Yes, that looks like powdery mildew fungus which commonly affects many plants in our gardens, including Monarda,when conditions are just right, though disease pressure can vary from year to year based on weather.
Here is our Powdery Mildew on Flowers page: 
https://extension.umd.edu/resource/powdery-mildew-disease-flowers/
Managing powdery mildew in home gardens consists of planting cultivars bred with some resistance to the disease and doing anything you can to encourage more light and airflow around the plants so that leaves do not stay wet for long periods of time. That could include thinning, watering early in the day, with water directed towards the base of the plants etc.  Sanitation can help as well- you can try removing the most affected leaves and those that have fallen to reduce the pathogen load in the garden (put them in the trash), but it may soon arrive from plants from afar. 

Are you a commercial or green industry professional? 
You could consider using a fungicide, but they are not curative- they only slow the disease down and need repeated applications, which we don't generally recommend for home gardeners.


Christine

Hi,

You can rehome them if you want, as powdery mildew is an extremely common plant disease that affects a wide variety of species, and its severity can partly depend on the weather any given year. (Powdery mildew on Monarda won't necessarily infect unrelated plants, but how likely it is to return in a future year is hard to say.) Even if this specific infection doesn't come back later, either after a cut-back or at another gardener's home, another new infection could begin that would look the same and be impossible to distinguish. (Nor would it matter much, as anytime it appears on plants, no action is generally recommended since fungicide might risk harming the pollinators the plant is often grown to attract.)

If you cut them back and keep them, they might grow back more mildew-free or might not...it's hard to predict, and spores can always blow-in from elsewhere and create new infections if the conditions remain suitable for infection (warm days, cooler nights, high humidity). Some individual Monarda plants, even among the straight species, could have higher or lower resistance to infection than others. While thinning them out either now or as they regrow may help improve air circulation to keep leaf surfaces drier to suppress infection spread, in this case, the plants don't look too crowded to benefit much from thinning, but it would not hurt (other than potentially reducing flowering) to try if you wanted to experiment.

Miri