Illness/Sickness: Red mark on head growing

[Montecarlo]

Hi
i've had Pumpkin for a several months now. She came from a pet shop with a red mark on her head. the photo is when I first got her. i've just noticed that this seems to be growing outwards. doesn't appear ill. should I be worried and what can I do? I have 2 axolotls in the tank ans both seem ok. hoping you can help!

 

[Otterwoman]

I don't know what this might be, hopefully someone else will pipe up!