Some powdery substance on roof rafters
It's just cellulose insulation dust from when it was blown in. It's identical to what's below. 

Your recessed can should be covered better, by the way. There should be as much above the can as there is above the ceiling elsewhere. You could also overlay some fiberglass batts.

Lee Sam says, "If the cans aren't rated for insulation contact (IC), I’d build a box out of gypsum board or wood that is a minimum of 2” away from the light on the sides and 6” away from the light on top. Then, cover the box with insulation."