What is this thing called? Replacing a tub spout

Hi, DIY here, trying to replace this galvenized elbow which is leaking on the shower side (threads are rusty and it leaks around the nipple when the diverter is used).

What is the piece called between the elbow and the mixer valve? is this what i want to loosen when removing the elbow? will it come free from the elbow and the valve at the same time? THe elbow goes through a mortar wall so it's not going to be possible to turn the elbow.

It's a threaded pipe nipple.
I like to pick up a brass 90 and brass nipples. The rest of the home is copper, why use galvanized?

The thing you probably have to change is the nipple between the elbow and the diverter spout, since the one from the valve to the elbow does not seem to be deteriorated.

you probably have to change is the nipple between the elbow and the diverter spout

Click to expand...

That would be the quick fix.
It's normally the spout, of the end of the nipple that the spout threads onto that goes bad.

While you're taking it apart to replace the rusting bits...replace all of the galvanized you can get to easily...it will then outlast the house. FWIW, that old style tub/shower faucet can be repaired, is grandfathered, but if it were to be replaced, would need to be replaced with one meeting today's anti-scald technology requirements. If you ever are in the shower and the water gets hot or cold from some other user, you may want to consider changing it out for a modern valve that doesn't have that problem. Old and young people have thinner skin, and are more susceptible to falls and burns, so at some point, this should be a consideration. They make renovation/remodel plates to cover the old holes and provide a platform to install the new valve cover onto (most are single handle).