@XxIcefire
First, I adore that name, Xipe Totec!
Second, I think that would be gill fungus. Now, before you panic, let me say a few things. Is this the first time you've encountered it and your axolotl doesn't have any of it on her? If the other one is spotless, I would honestly quarantine Xipe Totec for a bit. Have you tubbed your axolotls before? Do you have a safe place to do or even a spare tank to use?
Third, I have learned there is a tier list of what to do with this. Tier 1, non-invasive, meaning, wait and see. Tier 2, minimally invasive, quarantine or treat the water with them inside the tank. Then tier 3, invasive, and sadly, quite painful, the dreaded salt bath. I have heard and read a bit on the salt bath and everyone says it'll kill the fungus but it also hurts quite a lot.
@Calgarycoppers has a recipe for a mixture they use. It contains some salts within it that are like a preventative thing. They help. I am not sure about the specifics but if they reply, they can explain the solution better. All I know is how to mix it and that it helps.
Oh and fourth, final point. How is your axolotl acting? Are they bright, alert, and responsive? Still eating? Still going to the bathroom regularly? Not floating? If Xipe Totec is still eating, still coming up to the surface if you wiggle your fingers near the tank (showing alertness), then your axolotl is pretty good. My little girl has fought off 3 patches of gill fungus and has regenerated her missing filaments already and only once did we get close to a salt bath. Keep an eye on Xipe Totec and your other axolotl. You want to watch to see if they're scratching at their gills. I've seen mine do it with her hind legs, like a dog, but some people say they may use their front, 4 fingered paws. If you see that, you have an irritated axolotl. Eventually the gill fungus will fall off, either on its own, or the axolotl will remove it themselves. Don't try to remove it yourself, that hurts, a lot.
Sadly, I do not know how to cure gill fungus, and am currently dealing with a third patch on my girl's gills but she is behaving, eating, very alert, not swimming around aggressively, not scratching at her gills, so I am trying to treat it with simple things in the water. Nothing extreme.