The term Plus Alpha is term using loan words to mean something different in Japanese. It means "something extra", usually like a bonus. In this case though It was referring to Tsukuyomi being there as extra since she's actually not from their school. Typically, if something is a bonus freebie though, it'd be an "omake". But, Plus Alpha has been seeing more use in recent years. If you frequent websites like pixiv and you notice someone's work has plus alpha in the title of the work, that means there are some bonus images like rough sketches or alternative versions of images.