'Easily the Most Insane Animal Footage': What is This Strange Underwater Creature?
Initially it just whisks by, but keep watching for its return, where it balloons out to over 400% of its original size, then turns itself inside out. Its skin appears to be at least partially reflective (the light glares off of it) and it has this hexagonal membrane.
Edited: Second direct youtube link since the one above has been changed to private:
Allegedly a Deepstaria enigmatica -- appropriately named? If it is, this might be the first video of it, and the first live specimen recorded? This one appears to have only one arm, though the rest could be retracted up inside the bell.