The speaker takes a walk on sand that keeps shifting underfoot, and from that small, physical experience a larger unease grows. If the ground itself won't hold still, what can anyone trust? The flat statement "I believe nothing" lands hard because it sits alone, set apart from everything around it. It sounds less like a boast than a tired conclusion drawn from walking on something unreliable.
The last two lines are stranger. Eyes "can hurt the winter," as if looking were an act that could damage the season, and the winter is seen "in green spectacles," so the cold, bare world is tinted by a lens that makes it something it isn't. The poem seems to suggest that our way of seeing alters what we see, just as the wind alters the sand. Nothing is stable, either outside us or in our perception. The mood is quiet, skeptical, and a little uneasy, though the calm, spare tone keeps it from feeling panicked. The title, "Mutability," means changeableness, and the poem lets us feel it instead of explaining it.