If yu verify that the box is grounded by the bx cable then you can install a self grounding receptacle and no ground wire from the receptacle to the box is needed. These are specially marked on the box to be self grounding and are different than a cheapo receptacle. You can find them at Home Depot. Make sure that it has the gold spring clip on the yoke where the screws go through. You'll see one on the receptacle pictured below.
