Yes, that is a crack, or at least looks to be with about as much certainty as a pic can provide. That's a relatively common place for cracks to occur, with the thick square-edged boss creating a stress riser on the thin tube.
That the line of it continues into the area where a shifter or cable stop would cover the paint adds to my doubt that it's a scratch in the paint.
The only way to know for sure (unless you have access to an X-ray machine or ultrasonic tester from your side job ... say at Boeing or Airbus) is to take the paint off and see if it's a surface scratch or deep crack.
Since it's a metal bike, sanding it down won't hurt it and if it is a crack, your bike is toast anyway. Aluminum frames can't affordably rewelded like a steel frame.
Update: If you have balls or labia of steel and are willing to risk it (WARNING: STUPID ADVICE FOLLOWS) you could mark the endpoint of the crack with a sharpie and carefully monitor its progress while riding gently. If it lengthens or widens, it's a crack. If it doesn't, it still might be a crack. In any case, it'd be highly risky.