“The Rousalii’s themes are humor, protection, weather, fertility, fairies and growth. Their symbols are water, linen and green robes. A group of ill-treated women in life, these Goddesses often create mischief when they interact with humans, especially those with nasty dispositions. They do, however, have a good side. The Rousalii know the dances that make plants and people grow and thrive and sometimes they will teach them to humans. In literature, the Rousalii sometimes appear as water fairies, begging linen from passerby, which they use to make green robes for fertility rites.
In Romania, people would tell you that it is best to stay home today and leave the Rousalii offerings of bread and salt to avert their impish ways. If it’s windy, definitely stay home; this means the ladies are in a foul mood. To protect yourself, place worm wood under your pillow, pull the covers over your head and stay put! On a less drastic level, wear something green to keep them happy and try this spell to encourage the Rousalii’s growth or maturity in any area of you life: Take a little piece of linen (or cotton cloth) and dance with it in your hand, moving clockwise and saying,
‘The dance of life, the dance of power, Rousalii, join me this magic hour! To ______________bring growth and maturity; by your power this spell is freed!’
Fill in the blank with your intention. Tuck the swatch of cloth on your person, or close to the area that represents your goal.”
(Patricia Telesco, “365 Goddess: a daily guide to the magic and inspiration of the goddess”.)
In Patricia Monaghan’s book, The New Book of Goddesses and Heriones, I found references to these Goddesses as “Rusalki”. She states, “these Russian water spirits were originally human women who drowned or committed suicide. The company of naked, wild-haired Rusalki rose each spring from streams to beg bits of white linen from humans. The Rusalki hung them from trees after carefully laundering them. (One who accidently stepped on the Rusalki’s wash would be spastic thereafter.)
Their spring cleaning done, the Rusalki began their nightly dances that help plants grow and mature; sometimes for these occasions they wore long white unbelted tunics or robes of green leaves. Humans could lose their souls by witnessing the beautiful dances of the Rusalki, which usually brought rain to the growing plants. When summer was over, the Rusalki retreated to the feather nests at the bottom of the streams, where they hibernated until the next spring” (p. 269).
I also found that it is believed that “the Rusalka of Russian myths are the spirits of young women who were murdered before marriage and are then cursed to live in a lake in the form of a mermaid. There they will sing sweet songs to entrap men into the water and drown them. A Rusalka can be released from Her demonic form if someone avenges Her murder.
The Rusalka are slim with long, loose hair, blazing eyes and magnificent breasts. Their hair may be light brown, blond or green. They can assume the form of a fish or have legs like a human. In the latter form, they haunt the forests, dance with the moon and swing from the branches of trees. Often we see them sitting on the bank laughing with their friends the water sprites. Sometimes they visit local villages to join in the dances and entice men into their lakes to become their husbands or kill them.
If you would like to go swimming with the Rusalki (plural of Rusalka) put fern in your hair so they cannot pull you under and drown you. Some say that only witches can swim safely with Rusalki.
Another group of Russian myths claim that the Rusalka are water nymphs who marry the Wodjanoj. The Wodjanoj are male water spirits who live in great castles under the water and can change their shape at will.
Marriage alters the Rusalka. She goes from wild and lustful to sweet and demure.
I see many obvious links between the Rusalki of the Russian myths and the mermaids of Celtic myth. Both are beautiful, sexually liberated and occasionally dangerous. They are both descended from Goddesses of fertility and retain some of their characteristics. One article I read makes especial reference to the hair of the Rusalka. It is loose and uncontrolled like the Rusalka themselves. Notice how even now we associate loose, wild hair with sexuality.” 
Beautiful-mermaid-art.com, “Russian Myth: The Rusalka“.
Monaghan, Patricia. The New Book of Goddesses and Heroines, “Rusalki”.
The Bulgarian Festival Calendar, “Russalii or Roussalya: Wood nymphs (Samodivas) and water nymphs (Mermaids)“.
Tribe.net, “Rusalkie (Slavic water spirits)“.
Wikipedia, “Semik“. (“Green Week” – ancient Slavic fertility festival celebrated in early June).