The long term solution is to get rid of all the galvanized, as suggested above.
The short term solution is to replace the one defective part, the nipple. At this point, upgrading the replacement to brass seems a waste, but perhaps there's a reason for it.
You should not need to open the wall to do it. If the wall is caulked or plastered to the pipe, merely remove 1/8" or less of the material. Use tape and dope on the new nipple to ensure no leak in the hidden location.