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Cestum veneris
Long transparent gelatinous creature. About 50 cm in length. Moving through water serpentine like.
I think, you are correct :) after reading description it should be it. And since last few years we have more and more species migrating from Red Sea to Mediterranean it is possible.
I did think so, but it looked more like this http://species-identification.org/specie... (Velamen parallelum) but it was way to big for description.