Worms in the waterfall

While cleaning my pond this week, found a whole slew of worm-like creatures
living in my waterfall.

These worms were extremely small, probably under 1cm in length, maybe 1mm
thick, one end attached either to the stone, or embedded in some algae.
They'd stand on end, and move non-stop back and forth, back and forth.

Also saw some other creatures, about the same width, a tad shorter, that
were scooting around the waterfall at some fast pace.

My waterfall moves quite fast, I'm kind of surprised anything would take up
house in it.

Any ideas what these could be? I am hoping they are not mosquito larvae,
which I thought preferred standing water as opposed to fast moving water.

I have Robins, Grackles, and Mourning Doves that bath in the waterfall, also
want to make sure they are not at risk, or the depositors of said creatures.

Thanks.