What does nix/nixes mean?

That would be nix, nixie or nixies.

The Neck/Nixie (German: Nix/Nixe/Nyx) are shapeshifting water spirits who usually appear in human form. The spirit has appeared in the myths and legends of all Germanic peoples in Europe. Although in recent times such creatures have usually been depicted as human in shape (albeit in many cases shapeshifting), the English Knucker is generally depicted as a wyrm or dragon, thus attesting to the survival of the other usage as any "water-being" rather than an exclusively humanoid creature. Their sex, bynames, and various animal-like transformations vary geographically. The German Nix and his Scandinavian counterparts are males. The German Nixe or Nixie is a female river mermaid.