Yes, it's space junk: after about 6 hours, the second stage will stop working and there will be no way to change the trajectory of stage and payload. So it's a non-functional satellite, i.e. junk.
It's not in Earth orbit, so it's unlikely to cause a problem here. There is no propulsion system that will work after about 6 hours, and no sensors to detect an imminent collision.
If it reenters at some point (because its orbit intersects that of Earth or Mars), reentry will be fast enough that it'll burn up.