That's a Moon Jelly, isn't it - easily "recognized by its four horseshoe-shaped gonads," sometimes five. Isn't it harmless? Certainly pretty.
https://en.wikipedia.org/wiki/Aurelia_aurita
I think As I said somewhere in here, there were a S%^ton of these and then few here and there with the same shape, but hanging [-]testicles[/-].stupid autocorrect. Tentacles.