noun; A mood girl is a girl who is hooking up with a guy yet sometimes he likes her and other times he doesnt. Basically when he is in the mood to like her and hook up with her he will...when he is not in the mood to like her and hook up with her he wont. He expects her to take him back and their affections to remain the same. He also doesnt think of your feelings because you are in fact his "mood girl"
Jamisha and Chrinoid are hooking up. They talk everyday and hook up occasionally. One day chrinoid decides he doesnt like jamisha anymore and he wants out. Jamisha is devastated. A week later Chrinoid likes Jamisha again. They flirt then hook up. He is now in the mood to be with her again whereas he was not before. Therefor she is his "mood girl"