That is a flower stalk. Once that reaches near the surface, it will form plantlets along the runner. If the runner is along the water surface, it may or may not grow flower buds. If the runner leaves the tank, you will get little, white flowers instead of plantlets. I've had those runners grow 6+ feet long.
Once your plant starts sending these up, it will continue to do so indefinitely from my experience. They can be quite annoying to trim off all the time, but sometimes it's neat to see the flowers, or if you want more sword plants from plantlets.