淺淡灰色物質 浮 coolant表麵

When I open 水 箱(coolant) of my car, there is a few 淺淡灰色物質 in coolant of my car. 淺淡灰色物質 浮 coolant表麵. car is flush twice.
coolant is ok (green). Could you please let me know what it is and what happen for my car (toyota corolla)