Yes, that looks like the start of a fungal infection.
To treat this you need to do two things:
A Salt bathes,
B Fridge your axie.
The salt baths need to be done three times daily(if possible if not, just two) and use a container that is longer than your axolotl. Mix 2-3 teaspoons of uniodised salt per litre of dechlorinated water. Make sure you dissolve the salt, because salt can burn your axie if you just put them in on top of it. When you first put your axie in their salt bath, they will probably find it really irritating to begin with (remember to use a lid if it is a shallow container). Jim will need to stay in there for 10 - 15mins, BUT NO LONGER THAN 15MINS, soreally good idea to put a timer on for 10-15 minutes so you know exactly when to take him out. Empty the salt bath after each use and make up a fresh one each time.
You will need to put Jim into the fridge because the fridge can help with the fungus (slow down growth).
Jim will need to have his water changed daily with fresh dechlorinated water. You will need to keep bottles of dechlorinated water in the fridge, these will need to be used for the water changes to keep the temperature consistant for your axie so that she doesn't suffer temperature shock.
When Jim is in the fridge, you should also put a towel over his container to stop him getting stressed out over the light going on/off when people open the fridge.
While Jim is in the fridge remember, he probably won't want much to eat, so don't worry about that.
Because your axolotl needs to be in the fridge, make sure to use the dechlorinated water from the fridge in the salt bath, so it's the same temperature as the fridge water to avoid fluctuating temperatures which can be very stressful to the axie.
I would also continue with the salt baths for around 4-5 days after all the fungus has disappeared to make sure it doesn't come back.
Good luck!
AD: What is the temperature of your tank?
What are the ammonia, nitrite and nitrate levels?
Treating the fungus is obviously a good place to start, but it is important to try and work out why this happened in the first place (ie. temperature??) so that it can try to be prevented next time.