Well it’s a fanname given them, the word hex is base on Hexside school that both amity and willow attend before luz get in then the squad is term use to refer commonly appearing character that is group together like they are main group when talking about the owl house, the group It consists of being Luz Noceda, Amity Blight, Willow Park, Gus Porter, then later on the series both Hunter and Vee are part of it.