That’s a harmless fungal growth commonly referred to as “powdery mildew”. It’s most prevalent during periods of high humidity and cool nights. It also tells you there is a high nitrogen content but it is completely harmless to your lawn.
While a fungicide is available to treat severe cases, we at BAM! Green have never seen a case severe enough to warrant a fungicide treatment.
What to do? Give it a few days — weather conditions usually change. The grass blades will continue to grow and the infected blades will simply get cut off by your lawn mower. Powdery mildew is also a common problem in many tree species.