When we think of mood we refer to the overall emotion created by a work of literature. This emotion can normally be described succintly using an adjective or two. Let us consider what happens in this scene first before deciding what the mood is. This scene opens with the whispered conversation of the doctor and the gentlewoman who are discussing Lady Macbeth's condition and how she is sleepwalking and whilst sleepwalking saying things that the gentlewoman is not willing to repeat. Lady Macbeth enters and appears to be washing her hands in her sleep and confesses her involvement in the murder. As she exits, the doctor and gentlewoman discuss the state of Lady Macbeth but also of Scotland before the scene ends:
Foul whisp'rings are abroad. Unnatural deeds
Do breed unnatural troubles: infected minds
To their deaf pillows will discharge their secrets.
More needs she the divine than the physician...
The mood therefore seems to be one of menace and despair, as we see Lady Macbeth, who was formerly so resolute, break down under the weight of the evil that she has abandoned her self to, and we feel sympathy for her character. Likewise the way that her acts have not just impacted herself but also all of Scotland gives rise to a real feeling of menace, as we await the invasion of Malcolm's forces to liberate Scotland.
When this scene opens, the mood is tense and anxious. One of the Lady Macbeth's gentlewomen is waiting with the doctor to see if her mistress will sleepwalk again. The gentlewoman is very concerned about Lady Macbeth's behavior, especially some of the things she has said, since Macbeth went away to war.
Despite some initial skepticism from the doctor, Lady Macbeth soon enters and it is clear that she is indeed sleepwalking. At this point, the mood becomes more serious as the reader realizes the extent of Lady Macbeth's state of mind. Her suffering is clear: she washes her hands for as long as fifteen minutes, for example, and talks about the Thane of Fife's missing wife. In other words, Lady Macbeth is unconsciously confessing to the crimes committed by herself and her husband.
As a result, the mood is also tinged with sadness. The doctor admits, for instance, that this disease is "beyond" his skills and experience. Lady Macbeth is, therefore, destined to be forever plagued by her mental suffering that is clearly in an advanced state.