Make sure you have your removal tools ready. They're absolute masters of camouflage.
It's a damselfly larvae. Miniature versions of a dragonfly but still great underwater predators for all things small.
Once you decapitate them, they're harmless. I watched my barbs battle with a swarm of the nymphs. Barbs always win, but sometimes it takes a while for them to break free of their jaws.
They're essentially a underwater praying mantis until they turn into adults