Not sure what it is. An adapter, if you found one that worked, would make the inlet pretty long. I'd just remove it entirely (pipe wrench works best in my experience, ideally in conjunction with a bench vise) and replace. If you don't mind the color, a brass nipple and nut from Western Enterprises (AirGas is a WE retailer) should cost $10 or less.
That nut likely will slide down the nipple so you can get at the nipple. You may have to smack it on a hard surface or twist it down. Can't see inside, but is there a washer crammed in there?