Looks like caulerpa. It's a type of macroalgae that a lot of people keep in their fuges. Since it's just the one stalk, I'd say throw it out, as it can turn your water yellow. Chaeto is definitely the superior macroalgae, and caulerpa doesn't do anything that chaeto can't do, so you're not missing out on anything by tossing it. Chaeto also has the side benefit of it won't turn your water yellow.