Axolotl with black marks and strange lump

I thought that now might be the time to bring this up. My leucistic axolotl, Finnegan has had these black marks appearing along his back, along with a strange lump. I came here to find out information, and I read that it's normal for older axies to develop dark marks and the occasional lump, so I brushed it aside.

Today however, I noticed he was thrashing about a little, it looked like he was trying to twist his body to get at the lump. I was also concerned how big it was growing, so I separated finnegan from his tank mate to inspect it. I gently rubbed his side with a cotton bud, only to find the black marks were coming off (and he twitched sharply like it caused him pain.)

I feel terribly guilty that I should've noticed this sooner. Is there anything I can do to treat this lump and black 'mold'?

There's also an unrelated bump on his forehead that he's had for years. (image 3)

https://i.imgur.com/1INLejm.jpg
https://i.imgur.com/gbUxhIB.jpg
https://i.imgur.com/3mzGCK5.jpg

The short nub gills are a result of another slightly more aggressive wild type tank mate.
Finn hasn't had any trouble eating. He still eats regularly, albeit with a little less 'gusto' than he used to.

Bumping this.

I think the black is not as worrisome as the bloated looking chin. I wish an axie keeper would weigh in; I've never kept axies.

Bumping again.

Another bump.

The bloated throat is often a sign of bacterial infection , i havent a clue about the black marks.