The speaker looks at sea glass on a beach and thinks about how long it takes to make. Waves slowly wear clear glass down until it turns cloudy white, a change that might take fifty years. From there the poem slips into memory. The pale glass reminds the speaker of Grandma Dagmar's platinum blond hair in photographs from 1936, when she looked like the film star Jean Harlow.
The most touching detail comes at the end. Dagmar threw away broken milk bottles, and the speaker imagines she did it so that a grandchild might one day find the pieces and rebuild them into "a ghost or new moon." Whether or not that was her real intention, the speaker chooses to see it as a gift across time.
The mood is quiet, tender, and a little wistful. Sea and memory work in the same way here: both wear things down, but both also leave something beautiful behind. The speaker seems to feel love for a grandmother known mostly through photographs and fragments.