ok, sorry ... looked a bit like a goldfish in the picture
the picture is not very clear and I cant really see the infected area very well.
I had similar problem with some animals last year, but with my animals the fungal infection spread very very quickly, developed openings (almost like puss blisters), the animals developed red dots on the belly and quickly died .... these animals stopped feeding as soon as the fungal infection appeared and I was forced to force feed them.
But since your animal is still feeding, the fungal infection in only in the one area and doesnt seem to be spreading ... I really dont know what to tell you, I dont think your animal is in any imediate life threating danger but still yes you need to figure out what the problem is and take care of it.
You could try to use some betadine (which is similar to iodine), if you use this you could put some water in a tub or bucket (just enough so the water level will reach the infected area) squirt in some betadine not a lot just enough so that you can tell it is in the water, then soak the animal in the tub/bucket for about 10-15 minutes .... this would normally be done every day for 2 weeks.
other than that, other options would be medication by injection, such as amikacin or baytil ... but I think these would be a last resort!
oh one other option would be a daily soaking in potassium permanganate, this is used quit often to clear up fungal infection, with this again small amount of water in a tub/bucket enough so the water will reach the infected area, add in a small amount of potassium permanganate just enough so that the water turns a light pink, soak the animal 10-15 minutes a day for up to 14 days.
So you know, I am not a VET and I highly suggest you get the animal to a vet and not opt treatment on your own, but if you are forced to treat the animal on your own ... the options I listed above are the normal for treating a fungal infection
Good luck with your animal!!