It sounds as though your lawn has powdery mildew. This is a lawn disease that primarily affects Kentucky bluegrass, although some fescues can also get it. Severe outbreaks may occur on bluegrass growing in shaded areas at moderate temperatures and high relative humidity. The mildew usually disappears when environmental conditions change. It seldom causes injury that is severe enough to warrant spraying. If you’re patient, the problem will likely take care of itself. If it comes back repeatedly, you may want to consider growing a less susceptible variety of grass in the shady area, or plant a shade-tolerant ground cover there.