It looks like columnaris to me, also. Kanaplex doesn't help much with this disease. You need a different anti-biotic and you need to treat the whole tank. Columnaris is a gram negative bacteria. There are several drugs that can help.
I have success with ERYTHROMYCIN. There are a few fish meds that contain this and it can also come as that name. Maracyn, (now Fritz Aquatics' Mardel Maracyn). API offers it. Some say because this drug mainly treats gram positive infections
, something else should be used. So you may want to consider the popular one two punch of "the combo antibiotic treatment of Kanaplex and a nitrofurazone. If you cannot find Kanaplex, and alternative would be Mardel Maracyn 2.
The thing about columnaris is that it is noit just single variety. There are myltople strains of this disease which vary in lethality from the worst able to wipe out a tank in a few days with the fish never showing external symptoms. On the other end of the spectrum are varieties which can take months to kills fish. But you will see the external symptoms. Incidentally, it should be safe to conbine thee two Maracyns. Here is what Fritz says: "May be Used Along with Coppersafe®, Maracyn® Two, or Maracyn® Oxy"
https://fritzaquatics.com/products/...ts-by-disease/bacterial-infections-freshwater
I probably would have to say columnaris is the disease which I have had to deal with the most number of times over the years. I have only seen Ich twice. I have seen a few cases of wasting disease. I have pretty lucky with treating it. However, I have never had one of the more virulent strains which kill fast.
edited to add the following:
There are a number of antibiotics that can work agains columnaris. However, given the variety of strains and treating over time, some strains have developed resisitence to specific anti-biotics. Fow us as fish keepers who have no way to determine which srain out fish may have, we also do not know which would be either the most or least effective choice. This would argue in favor of the dual drug approach. I have never tried this however, so I cannot offer a first hand opinion.
Since this is only on one fish now and it isnt showing signs of being very ill, you might try using a hospital tank and treating just the one fish, A smaller tank means less medication is used. If you see other fish in the tank showing symtoms, this would change things, I would them return the fist to the main tank and teat it.
Columnaris can present in multiple ways, It is often called sddlenack disease because it startes as a white patch aon the fish at the base of the drorsal. It can also present as white patch on the body of the fish. I can also present in mutilpe locations. So you fish with the "mouth fungus" may develop white patches in its body or at the base of the dordsal.