So did you guess what it was then? Apparently Deepstaria enigmatica, is a jellyfish first described in 1967. The bell of this jellyfish is very thin and wide (up to approx. 60 cm), and resembles a translucent, folding sheet or "lava lamp" as the animal moves.
Or what I first thought was a large plastic bag or crisp packet! I don't think marine biology is my strong point.....ahem.
The older you get, the stranger things become.