Three short lines hold a small, odd scene. The speaker looks at a glass, perhaps a window, a mirror, or a drinking glass, and sees the moon reflected there, as if it were dancing. The title, "Terpsichorean," means relating to dance, so the moon becomes a performer. But the speaker imagines it is angry, and the cause is a "crooked foot."
It's unclear whose foot is crooked. It might be the speaker's own, so the moon seems to scold a clumsy partner. It might also be the moon's reflection, warped or wobbling in the glass. Either way, the speaker feels awkward, imperfect, and a little judged, even by something as distant as the moon.
The mood is playful but slightly uneasy. There's humor in picturing the moon as a fussy dance teacher, yet there's also a quiet loneliness, a sense of being watched at night and found lacking. The poem leaves that feeling unresolved.