I’m not quite sure…
What is in the pellets and what brand are they?
Also, if you’re using pellets, I suggest using axolotl-specific ones instead, such as A Lot’l Axolotls, or if you can’t find specific pellets, you can also get sinking carnivore pellets, such as Hikari Sinking Carnivore Pellets.
These two brands have caused no problems for me over the years.
Do the lines move on their own?
Sometimes, certain fish foods can introduce waste eating worms into your tank.
Of course, there are also species of harmful worms that can stick to the tank glass, so I’d look into it.
If the lines are moving by themselves, I’d suggest taking your axolotl out of the tank and putting them in a temporary holder, like a large pot, or even better, a small fish tank.
during this time, you can research the lines a bit more and see if you’ll need to take any course of action against them.
Hope this helps!! -Torro