OK, first of all, stop watering in the evening or at night. If you water at night and get the leaves of the plant wet they will take longer to dry thus providing an extended period forfunal diseases to take hold. Water in the early morning instead. Also use a watering wand so you can soak the soil without wetting the leaves. Wet leaves are just what fungal diseases need to get started and thrive. What you describe is a fungal disease called powdery mildew and it is very common to see this disease attacking bee balm, lilac, garden phlox and some other plants. That’s the bad news. The good news is that the powdery mildew on your bee balm is specific to bee balm and won’t spread to your other garden plants nearby unless the are also bee balm. Once powdery mildew starts, you have no choice but to spray with a fungicide. Since you know that you have this issue now, you can expect it next year as well. You could start spraying or dusting the plants with the organic fungicide Garden Sulfer about once a week or after a heavy rain. Rains washes the sulfer off the leaves. Since the disease has already started, you’ll want something stronger. For this I suggest Bonide’s Infuse fungicide. It is a liquid spray and you want to apply it every 10 days or so following the directions on the package. Remember, these sprays are most effective when used as a preventative before the symptoms show up. If you like bee balm then you should start spraying by early June and continue through the the first half of August.