If you start to see dark grey matter appear on things, it is from aluminum vibrating on something. I had a plane with a muffler that didn't touch the plane when static, but when running it slightly rubbed on a little corner of the plane. The result was the baffle in the muffler coming loose and other parts starting to wear from the vibration.

Running your nice engine while making contact with the plane is not the best idea. Maybe it won't lead to serious damage, but for $6 you can avoid the possibility.