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. An object whose course cannot be controlled, and a potential future navigation hazard.
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), reentry will be fast enough that it'll burn up. If it entered Mars' atmosphere, it might survive reentry and crash on the surface.