That's a damselfly larva. It'll eat small shrimp, but doesn't get big enough to endanger any fish other than newborn fry. Probably came in either on plants from outdoors or as an egg laid by a damselfly.
Dragonfly larvae get large enough to threaten fish, but are much fatter. That's not one.
To find it, check the plants, especially in less well-lit areas. Sit quietly and watch for movement, they're lurking predators.
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My current project, a 65 gallon aquarium stocked with vernal pool fauna.