I watched the video yesterday, but now it's switched to "private". I wonder why.
Anyway, a German article (_http://grenzwissenschaft-aktuell.blogspot.de/2012/05/tiefseekamera-filmt-sonderbares-wesen.html) talks about how scientists don't think it's a placenta:
After several assumptions that it is a sunken placenta of a whale, the team of "DeepSeaNews.com" succeeded in identifying the strange creature as a rare jellyfish. The researchers said that a whale placenta probably wouldn't be able to sink so deep without being strongly eaten away at by other animals.
The animal is a deepwater jellyfish by the no less mysterious name of "Deepstaria enigmatica", a subfamily of the "Fahnenquallen", which were first discovered in 1966, 723m deep close to California. This identification has already been confirmed by Dr. Steven Haddock from the Monterey Bay Aquarium Research Center.