Slime Girls is a unit under Alice Project. They are considered the first group that complete beginners go into who have had no past experience singing or in the entertainment industry. They are called "Slime" as in the lowest level monster in most RPG games.
Since Slime Girls is a trainee group, they aren't promoted as heavily as the Kamen Joshi groups. In some cases, it's hard to keep track of member line up changes.