русалка, the etymology is controversial), plur. “rusalki” (русалки) — female spirits associated with the water areas in slavic demonology, perhaps, one of the most well-known and complex сharacters in slavic folklore.
povodni mož - "Underwater man" is a spirit of the waters. Every slovenian river had their own they made offerings to, so that he would be calm and not give a hard time to people when they would be fishing for example. Each spirit of different rivers or lakes has usually their own name of adjective, most famous is Gestrin from the east, Povodni mož from Savinja who was the Master of Fairies and from Ljubljana.