Is this a weed?
We are not entirely sure which weed this is from your photos. Does it ever bloom? (Smartweed would have small stems of pink dotty-looking flowers. Japanese stiltgrass is a grass with a white vein running just off center down the leaf.)
Either way, you should be able to pull it- both come out easy. You could also use a non-selective herbicide containing the active ingredient glyphosate to kill it back to the roots, following label instructions carefully.