You most likely have a native Leptomedusae or Gonionemus vertens
Gonionemus vertens L. Agassiz, 1862 is a Limnomedusa with a narrow velum and many tentacles. It is apparently not established in Nordic waters, but has been recorded occasionally (Jensen & Knudsen, 2005). For more information see the Marine Species Identification Portal.
For more information on Leptomedusae see the introduction to Ctenophores.