It is moisture on the rotors. happens on every car I have ever driven in once a large amount of moisture or snow gets on the rotors. Happens to if you let your car sit out for awhile in high humidity/rain the first time you hit the brakes...in that case it is rust brushing off your rotors.
-Matt
PS yes it is safe so long as you are expecting it. Some vehicles like a few Mercedes models detect rain/snow and will actually lightly apply the brake pads during driving to keep them cleaned of moisture. I am sure this adds up to a lot of additional brake pad wear though (and decreased fuel economy)...even if the pressure is very slight.
-Matt
PS yes it is safe so long as you are expecting it. Some vehicles like a few Mercedes models detect rain/snow and will actually lightly apply the brake pads during driving to keep them cleaned of moisture. I am sure this adds up to a lot of additional brake pad wear though (and decreased fuel economy)...even if the pressure is very slight.