What makes you think the "C" is on the rock rather than the photo, on which it is so nicely centered horizontally?
Never mind - just saw that the image is an extract from a larger photo.
I was open to NASA's explanation on the 'C', but your suggestion was not their answer. (By the way, only the cropping might make it appear centered.) I found the claim of it being a coil of hair when it could not have been wanting and convenient, inasmuch as it implicitly kept any original evidence from being scrutinized.