Fossil Martian Bacteria or Algae
That is quite possibly what can be seen in this image, many times over, linked chains of cells, including some preservation of cell walls. Sure, it could be some exotic inorganic process - I can't prove that it is not. But the simple explanation is algae. I don't know if this is the rock "Russet" that they mention taking one micro of on sol 381, or not. Context?