That is definitely fungus. How old is your axolotl? And what are your tank parameters, including volume of water and temperature, please.
Oh and I would do more than 20% water change. I do about 50% at least once a week, and once a month I do a 75% water change. I take my axolotl out and put her in clean water, and she just chills with an air stone and some food while I clean. I went through a bad gill fungus with her in October and she came out well, but we had to do salt baths. I do not recommend you jump straight to that. Patience first, then bigger volume water changes, and if there is no improvement, perhaps a salt bath regiment. Again, DO NOT DO THAT YET. If your little guy doesn't improve in like a month, I can help you with the salt bath thing, I did it for a whole week. I've cried a lot in my life, but that week was one of the hardest weeks. You really gotta be careful cuz it is not pleasant.