It sounds as though your bee balm (Monarda didyma) suffers from powdery mildew. This fungal disease can be severe on bee balm. Plants under stress are more likely to contract the disease, so keep the soil evenly moist, and provide good air circulation to minimize the problem. However, if you're growing a susceptible cultivar, there may be little you can do to prevent your plants from turning gray by the end of summer. A better solution may be to grow one of the resistant cultivars, such as 'Colrain Red', 'Raspberry Wine', 'Violet Queen', 'Petite Delight', or 'Marshall's Delight'.