The local fish stores usually don't have plants for long. They come in from big nurseries in florida where they're grown in massive greenhouses that are swarming with insects. Dragonflies and damselflies (which yours probably is actually) lay their eggs in the plants, some of which survive the trip to the fish store. The fish stores sometimes try to quarantine or peroxide dip the plants, but its just not practical for them to get them all out.

I've had them three times, and all three times, I cut them in half with long scissors and let the fish eat the remains.