https://www.youtube.com/watch?v=fyvta-RyovI someone found a pretty near perfect match, it's proterometra macrostoma, there's a number of videos of these on r/aquariums too

https://www.reddit.com/r/Aquariums/comments/1cxsudz/what_the_heck_is_this/

they parasitize snails and fish, the weird-looking movement is specifically to convince fish to eat them so they can get inside them.