That looks like a galvanized pipe nipple. If it is, the better thing would be to replace it with a brass one. Regardless of the fact it is galvanized, it will rust after awhile giving you grief. THey use galvanized not because it resists rust, but because it is cheaper than brass which will outlive both of us. So, it's hard to tell for sure, and if you unscrew that pipe from the fitting in the wall, you may not get a brass one in without tearing up the wall (it might move when you remove the nipple), you have two choices, cut the wall open a little bit to see what's there and then fix it, or just unscrew the existing valve from the nipple, screw a new one on, and hope. You will need to use two wrenches/pliers - one to hold the pipe sticking out and the other to remove the valve. You don't want to loosen the nipple's threaded connection in the wall or it may leak.