Use rock salt, not sea salt. Rock salt is just plain sodium chloride. Sea salt has sodium chloride and a heap of other minerals in it. Rock salt is the same as aquarium salt or swimming pool salt.
The growth is not pink or red so it does not appear to be the gill filaments, which are normally red or pink, depending on the health of the fish.
The growth does not look like a fish louse or any sort of parasite, these are usually grey and don't look like that.
If it is responding to anti-biotics (Kanamycin), then maybe it has a bad bacterial infection in the gills, but I would have thought it would be red or pink if the gill filaments were involved.
Just watch out with anti-biotics because they can wipe out the filter bacteria. You also need to clean the tank and filter thoroughly before adding another dose. Ideally anti-biotics should be used in bare glass or plastic containers so the medication works on the fish and is not wasted on the bacteria in/ on the gravel, wood, plants, glass and other items in the tank.