Anyone know what these critters are?

[Sushiducks]

They still to rocks in our stream and waterfall. We brush them off but they keep coming back.

Thanks

Edit:stick to rocks

 

[j.w]

Black fly larvae. Fish like to eat them.

 

[Mayank]

Black fly larvae? Then you'll have pupal cases (look like mini long snails) soon as well. They usually twitch.