Say hi to the Portuguese man o' war. It's purple, it's clear, it looks like a balloon, it's painfully venomous, but it's not a jellyfish and you wouldn't have to convince me too hard that it's from an alien world. This Portuguese man o' war, which other than being a fantastic name, washed up on a beach to freak people out (and to expose the alien hideout that is the ocean).
The Portuguese man o' war is often mistaken as a jellyfish (because jellyfish look freaking crazy and we're more understanding of crazy looking things like the man o' war as long as they're called jellyfish) but is actually a siphonophore. Siphonophores are fascinating because they look like a single organism but they're not, they're actually a colonial organism made up of little individuals called zooids. Zooids are attached and dependent on each other so much that they can't survive independently.
What I have determined after learning all that and seeing the man o' war is that aliens have already crash landed on Earth and they've been living in the ocean. It's the only thing that makes sense.