You have a Nemopsis bachei
Nemopsis bachei L. Agassiz, 1849 (=N. heteronema Haeckel, 1879), originally from Atlantic coast of North America, although this has been questioned (see Wolff, 2005). Polyp solitary, small inconspicuous (Schuchert, 2007), and it is uncertain whether both polyp and medusa have been recorded in Europe. Has been found in the UK (Hebrides) and Norway, and apparently is established in the Netherlands (Wolff, 2005). See also Introduction to Ctenophores (and Cnidarian medusae).
Drawing of Nemopsis bachei or N.dofleini - they are almost identical (from Schuchert, P. 2007)