Hi, stottlar1. I've had exactly the same problem w/p.m. on the monarda I started from seed years and years ago. It differs from year to year depending on the humidity and rainfall levels. There's no cure for it. Just measures you can take to thwart its progress.
-Better air circulation.
Can you prune any plants around it? Can you move the bee balm itself to a more open area?
-Improved sun exposure.
-A pre-emptive strike.
An application of a cornmeal spray w/some cornmeal sprinkled at the base of the plant; some organic horticultural oil or some organic fungicide like Organicide or Serenade. (You can find them at many of the Big Box stores.) The key w/p.m. is to catch it before it manifests itself.
-Grow mildew-resistent varieties
. "Marshall's Delight" for instance.
For now, what I do is to deadhead and cut back to the nearest budding axil. If the plant isn't completely defoliated by my having to remove the p.m.-riddled leaves, I don't mind leaving it if it means I can get a secondary, albeit smaller, bloom. But you can choose to cut it all back now to its basal leaves if those powdery-gray leaves are really yucky looking. Otherwise, wait till fall and do that cutting back. Then make sure
you do a thorough cleanup underneath and around the plant. Remove and dispose of all mulch and the top 1" of soil. Don't compost it! Replace w/fresh, clean mulch and then in the spring apply some fresh, clean compost. Just as you'd do w/a tomato plant prone to early blight, remove the lower 7 or 8 inches of leaves to make it harder for the p.m. fungal spores to splash up on the leaves from the mulched soil.
There's always next year and p.m. or not, it's worth it to see such happy bees.