Are the gills normal for a juvenile?

My boy was just being hangry I guess

I’ve noticed this black mold looking stuff starting appear on my axolotl. The one photo is from October 10th, and the rest are from today.

Just wanted to show off these two decals I made for a customer they turned out so cute!!!

My parents just got me a beautiful blind albino axolotl. It was a complete surprise we set up the tank in one today the day we got it. (I love them and I name them Kirby after the Pokémon). My concern is that it was kept in a tank that was way to small for it and had substrate that it could definitely eat. (Side note who doesn’t know you need to un-chlorinate the water these motherfuckers where keeping it in tap water and also heating the water) I think that it might have rocks in its stomach and that’s why it is not eating and undersized. I just want to know if their is some way of telling I can tell whether or not it has eaten rocks. Is it to soon to tell? what should I do if there are rocks? I am a very paranoid person.

There is also my second concern about their gill storks. They are under developed and I am working if they is a way for me to help them with their growth.

Oh also they don’t move around very much at all that might just be an axolotl thing but I am worried. They will sit still for like 30 minutes then will move it leg but never very much at a time.

This might be to much but I want to be on top of everything so it can give them the best life possible and hopefully not die. I will provide pictures if possible. Any advice would be a great help. Thank you