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if it moves lke a catipillar and doesn't have legs its probably a fly larvae. There are many of flies with aquatic larvae. Keep it away from your fish. Almost everything that looks like what you have there is carnivorous and predatory.
It COULD be a dragonfly larva. I got some water lettuce and pulled several of something similar off. Then a couple months later was watching a documentary and saw dragonfly larva and thought that it looked just like the caterpillars I got rid of