Sadly, where there's one, there's usually two or three! The adult damselfly lays a cluster of eggs on the plant leaf, which then go into your tank and some hatch out. They will definitely take out fry, shrimp, perhaps a slow or weaker adult, so worth spending a lot of time sitting by the tank hunting for them and whipping them out ASAP!
This is one of my worst nightmares, sorry you're experiencing it! I've caught them in my pond before, and let them go back in since they're part of the ecosystem in the wild, and the adults are beautiful. But in my aquarium? Heck no! They kinda look like evil little monsters too, don't they? Same deal for dragonfly nymphs.
Danger: May consume fry.
How it gets in your tank: Plants, Live food, damselflies laying eggs in your tank(unlikely if indoors).
Size: Often ≤ 2inch
Larval stage of damselfly life cycle, are [acronym="Insects (Class Insecta) are the biggest class of arthropods and the only ones with wings. They are the most diverse group of animals on the planet. They are most diverse at the equator and their diversity declines toward the poles. With over a million described species—more than half of all known living organisms—with estimates of undescribed species as high as 30 million, thus potentially representing over 90% of the differing life forms on the planet. Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans."]insects[/acronym]. Have extendable jaws. Live underwater for several years before turning into adults. Have a three pronged 'tail'. Link, Identification Help.”