Two small images sit side by side here, and the poem's quiet trick is how they echo each other. In the first, a sheet of ice is imagined as a trap set for the moon, as if the frozen surface were waiting to catch that bright, distant light. In the second, the speaker admits that their left eye is troubled with rheum, the sticky discharge that gums up an eye and blurs what it sees.
The title, "Blear," ties it together. Blear means dim or watery, and it fits both the cloudy ice and the clouded eye. The speaker seems to be looking at a cold night through a faulty lens, so the moon is smeared and half-caught, much like their own sight.
The feeling is more weary than dramatic. There's a small, wry humility in moving from the grand moon to something as unglamorous as eye gunk. The mood is cold, hazy, and a little rueful, with the body getting in the way of the view.