Skin bubble in gills?

[sherrisixxx]

Hi there, hope everybody is well! I just wanted to check if anybody knows what the lump is in my axolotls gill? It is not fungus, but more like a bubble made of skin or something? It has been there for quite a while,and only very gradually getting slightly bigger, so am not sure if its just because he is very old! He is over 14 years old ( that's over 100 in human years!) and I have had him since he was a baby.

I have a feeling this might be something to do with his old age, but just wanted to make sure, in case he needs treatment. His water readings are fine,as is his temp.

Thank in advance.Have a good day!