There’s a straightforward double meaning (ambiguity) in this line. The word “mood” can mean something internal to a person:
> **mood, _n._ , 3.a.** A person’s humour, temper, or disposition at a particular time.
or external to them:
> **mood, _n._ , 3.c.** The pervading atmosphere or tone of a particular place, event, or period.
>
> _Oxford English Dictionary_.
So we can read the line two ways: the “dust of snow” shaken down by the crow has changed the atmosphere of the place, and also changed the speaker’s feelings.
Ambiguity does not fall under the remit of “figure of speech”, which usually encompasses devices in which words are not to be taken literally. But it is an important poetic device nonetheless:
> The machinations of ambiguity are among the very roots of poetry.
>
> William Empson (1930). _Seven Types of Ambiguity_ , p. 3. London: Chatto and Windus (1947).