Hey Elena thanks for your reply. Feel free to nitpick away, I only want the best for these guys and if I'm doing something wrong I'd rather know about it.
The male came with the female from the same owner, same tank. The tank was home to five in total and none of the others had anything similar. "1 female gold with lump on back behind head from injury when she was young (fully healed)" This was all the information I have about it.
Unfortunately I haven't seen anything like that before, and can't guess too well at it without knowing exactly how it occurred. Some axolotls do have mysterious growths, and that does not look like any parasite that I know. I'd say monitor the condition and get an idea of how long it's had it. If it is generally healthy otherwise, I'd call it benign. You could take the axolotl to a vet and they'd be able to sample it, but I'm guessing if the axolotl has had it for quite a while and if it really was grown from an injury, it's not anything to worry about. The axolotl does not look to be in too bad of condition otherwise.
Monitor it, and if all remains well with the axolotl and she eats well, then don't worry too much about it. It would be nice to get a vet's opinion, however. To me it just looks like a fatty mass of tissue. The red portion at the bottom of the ball might have something to do with blood circulation stopping there at the fatty mass, but I'm just making guesses at this point.