Alright, I have been scouring the internet and cannot find ANYTHING that looks like this. Except terrestrial lichen. Growing colonies of white cup-tipped... something. They grow on any surface, including directly on java moss, but seem to like the dead roots of Azolla that have fallen off. I dumped some snail eggs on the biggest colony, which immediately hatched: no effect. Neither seemed interested in the other and baby snails have all gone to greener pastures.
USB microscope just came in, so you get a closeup. The video is the most interesting: You can see water currents in the top of the frame from the beating scillia on surface of each of the cup-like tips. The needle is 0.36mm dia for scale. The twin brown threads leaving the frame are dead azolla roots. There is a flatworm that was investigating the center, but didn't seemed bothered at all by the tips. Assuming hydra have nematocysts, I don't think this is a hydra.
In case it isn't obvious, buy your aquatic plants from reputable sources...