10-50 to 6-50 wiring
Assuming that cable is in fact 6 AWG, that cable was always correct for a NEMA 6-50 receptacle.  It was always wrong for a NEMA 10-50. 

It really matters.  Some people have seen the inside of a panel and see where neutrals and grounds land on the same bus.  That's not quite what's happening. Neutral and ground serve very different purposes. One returns current normally, the other one is a safety net that saves lives. They are supposed to be kept totally separate, with equipotential bonding created between neutral and ground in one place - the main panel.  However in a simple residence, there is only one panel, so they allow the shortcut of simply putting them all on one bus.  That's all. 

The only wire relabeling that is permitted is a white wire in a cable can be relabeled to be a hot, so what you have done is correct.  This is the normal way of powering NEMA 6 circuits using common multiconductor cable. 

Being a modern house, if this was a range circuit they would have already used /3 cable and your range receptacle would already be a NEMA 14-50.  So I gather this is in a utility space and is intended for welders, kilns, EV chargers and the like.  A great many of those work fine without neutral, so you may never need a neutral. If you did, you'd need to replace the entire cable. You cannot retask that green wire to be anything else.  You also cannot use the MC shield as a ground path, so you do need the green wire.