The mood in the first act of The Crucible is one of fear and suspicion more than anything else. We're introduced to a world in which sin is condemned and often punishable by death, and in which pointing out the sins of others is totally acceptable and even encouraged. The girls are trying to escape punishment, so there is fear; some believe there is an outbreak (or infestation) of witchcraft in town, so there is fear; Tituba fears she will end up taking all the blame for this incident--again; and one (Reverend Parris) is afraid his enemies--also known as his parishioners--will blame him for this problem. More fear.
Suspicion is found in Goody Putnam who believes some women in the town are killing her unborn babies; in Thomas Putnam who thinks everyone is trying to take advantage of him; Reverend Parris suspects John Proctor is leading a "faction" against him; Reverend Hale suspects Satan may have a hold on Salem.
The result of this fear and suspicion is accusation, which ultimately leads to the deaths of nearly thirty innocent people.
If we define "mood" as an emotional state that is created in a work or a context, then I think that one of dread would be a good way to explain and identify what is present in the First Act. There is an emotional sense of dread when we open to see the praying over Betty. Yet, this turns into a more profound sense of dread when we see that all of this is concocted and is false. Dread is the emotional element present because only more bad can arise from it than anything else. When Abigail tells the girls to be silent and to essentially "stick to the plan," dread becomes highly evident because we fully understand how evil these girls are and how they are willing to take anyone down in the process. At the same time, I think that further dread is created when we understand Abigail's true motives toward John and how he resists these advances. It becomes a feeling of dread that is created because one of them is going to lose- either he or she. This means that even worse things are going to unfold and as the hysteria mounts and more names mount, the reader feels dread on both political and personal levels.