Identifying Fluffy Whiteness On Neon.. Fungal Or Bacterial

Hi,
one neon had a little white "fluff" develop some time ago on his side, I assumed at the site of an injury. I did try treating the tank with no success (with what I had in, I cant remember). Anyway, it has since moved to his mouth... googling the white fluff I kept finding it could be either fungal or columnaris (bacterial) is this right?
Essentially, I couldn't thoroughly decide which it was so have come for advice. I have moved him to a quarantine tank with about 10l in. and have treated with Kings Fungal & finrot treatment. This doesn't seem to have made any difference.  He is still eating but cant pick food up from the floor.

Any advice on this - and treatments?  If I do see improvements will this be a very slow process or is it more rapid than that?

I have Myxazin so will switch to that - have had a couple of days on nothing so will give him a wtr change and try that. I will get in some eSHA 2000 as stock too. have read positives about that one too. always gppd to have something already in.

there are 6 neons, and a boisterous single guppy (spends more time keeping others away from food pellets than he does actually eating!!) but nobody specifically picking on him!

The water is hard, I do a weekly change of about 12/13 litres (the amount my bucket allows!) out of 64litre tank. I do try to keep the water "clean" as prevention is far easier than cure!!

I will cross my fingers and give a good burst of Myxazin - do these things make slow improvements or can they be slow to respond?

thanks Sue