I know what piece you're referring to but anything below the nipple can't be replaced. On the "out" side of the tank the nipple screws into the tank and that can be replaced. On the water "in" side the nipple is connected to a copper line going into the tank. That piece cannot be replaced.
Yes, I agree with Rick, that's the water heater piping coming out of the water heater that's connecting to your coupler. If you have a pinhole leak from the pipe coming out of the water heater, you'd need a new water heater.
You know....it hasn't leaked for about 10 days now. I've turned it off at the spicket/spigot usually after using hot water but the towel at the spot has been completely dry. I know this doesn't mean...problem solved...but it has given me time to try and fix. Still need ideas of repair if possible before replacing whole unit. Thanks