浅淡灰色物质 浮 coolant表面

When I open 水 箱(coolant) of my car, there is a few 浅淡灰色物质 in coolant of my car. 浅淡灰色物质 浮 coolant表面. 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)