Three short lines give us a quiet scene: high in the mountains, just before the full moon sets, the speaker notices one more grey rock. That is all that happens, and the poem seems to want it that way.
The word "another" does a lot of work. It suggests the speaker has already seen many rocks, so this one adds nothing special. The moon is about to disappear, which hints at a moment of passing, yet the poem doesn't make a drama of it. Something grand like a full moon is set beside something plain like a rock, and neither is ranked above the other.
The feeling is calm, maybe a little tired or resigned, but also steady. The speaker doesn't seem to be searching for meaning. They simply look and record. The mood is stillness and a kind of patient attention, with the vast mountains and the small, ordinary stone sharing the same pale light.