[I got the rope]

Could be just a high contrast from the black and white pic? Doesn't the spectral analysis rule out salts? It looks like almost the same consistency as the dust from the surface, but with a vastly different color which could be enhanced by the black and white photo. I wonder what the color pic looks like?

[Djarum]

My speculation:

It looks like the rover drove over the pile of dirt it excavated, leaving a white track of the substance that was stuck to the wheel.

How did they manage to end the dig at nearly the precise point where the white substance starts?