If it is a fungus, a low concentration of Methylene Blue is your friend. As a matter of fact, there are three OTC medications that are every amphibian keepers best friends in the whole wide world.
Methylene Blue: Used to treat fungi, and ectoparasites
Tetracycline: Kills off most strains of red-leg
Panacure: Kills off most internal flagellates, cestodes, and nematodes. Especially important for wild caught specimens.
The salt baths dont seem to be working. I would recommend a low concentration of Methylene Blue, say... 3 mg per liter for five days or so. Not only will it stain the fungus, and let you see just how far the infection has spread, but it will kill it as well, without harming the newt.
That said, be careful not to get any in your eyes. It wont hurt, but you may end up having an urge to wander around in a desert riding giant worms and setting ambushes for people calling themselves "Harkonnen"
Oh, and this also where I say "I am not a Vet. I am just a biologist with a lot of practical experience fighting the usual infections that happen in large amphibian colonies. If you can, you really should take your newt to a vet"