Weird lump on my axo tail

[Milie]

Hello, I am new here. I adopted my axolotl last december and he came with kind of a lump on his tail. Its hard and now it openend. Does anyone here would have an idea what it could be and what should I do? He is still active and have a great appetite. Thank you in advance.

 

[axolotl nerd]

pictures aren’t wonderful, but i assume some sort of tumor - nothing to really worry about because it can’t be cancerous