Looks like jellyfish mesoglea. Basically the sea and surf pound away at the jellyfish body, pulling off the tentacles and obvious feature and just leaving this jelly-like substance behind.
Perhaps part of a Moon Jellyfish (Aurelia aurita) or Mushroom Cap Jellyfish (Rhopilema verrilli)
You can read more about mesoglea here: http://en.wikipedia.org/wiki/Mesoglea
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