Looks like a jellyfish.

http://globalspecialeffects.com/flogos.aspx

Essentially a soapy type foam filled with lighter-than-air gas.
This could be an experiment with the concept although it appears to have structure more like a super lightweight plastic bag. I wish there were a better idea of scale, as well as more video. Only one video/photo posting suggest it was smallish or perhaps a hoax. I suspect the effect could be duplicated with a combination of foam, and super thin bags, and a bit of helium. Check out "cluster ballooning" or "trash bag ballooning" with hot air or helium for an intro to the sort of devious creativity being explored.
If I saw such a thing descending, I'd try to chase it down as it appears to have been on it's way down less than a block away.
High altitude balloons are made of extremely lightweight material which usually burst at a given altitude, but you never hear of the remains being found or tracked.

Check out a somewhat similar "foam" encounter-

at least that seems to be an explanation in the you tube comments.