Fungus grows in cold water. Because axolotls are cold water amphibians, they are susceptible to fungal infections. Some have weaker immune systems too. If you haven't already, add holtfreters solution to your tank too. If you have other axolotls in this tank, you'll need to quarantine him to prevent spread. It is not necessarily always a water parameter issue, but it's also recommended you check the water parameters too when this happens.