They look like worms or larvae, stuck to the rocks of the waterfall. They will also collect on a leaf if it falls in: when the leaf is taken out they squirm around. Bio filter and waterfall is on 24/7. They can't be mosquitos in moving water?
Does not look like mosquitoes to me. Mosquitoes have a wind/unwind/spring-like action to them when they move.
I believe those are black fly larvae. whenever I see them in my pond I just knock them loose and send them downstream for a fish snack.
Thanks! The fish love em!