nubbsgalore:

baby fish hiding under the bell of a jellyfish and using its stinging threads as protection from circling predators. this “floating safe house” will provide the fish with protection and food until they’re big enough to venture out on their own. (source)

Those “leaves” around it in the third gif are fish as well! Cornetfish (not a typo, not cometfish) imitate strips of seaweed in order to avoid predators. 

There aren’t any stinging cells up under the jellyfish’s bell, so they hide under there, which keeps predators from spotting and eating them. I mostly see what look like baby jacks, plus a couple of filefish in that second gif.