Is it a growth, fungus, or parasite on neon tetra?

One of my neons has been hiding and not eating for a couple of days. I just finally got a glipse and "he" appears to have some sort of growth, parasite, or fungus on one side of his body near the pelvic fin. Swimming seems to be okay, but like I said, no eating and hsi mouth is moving quite a lot like he might be gulping. He stays low in the tank though and the other fish seems to be fine. I can't get close enough to this fish to see what the abnormality is. Sometimes it looks like a growth and sometimes like the "fuzz" of a fungus (but it is only in one small area). It does appear to be external more than a bulge under the skin. Any ideas? Serious responses only please. Water is due for partial chage tomorrow but parameters all seem to be within bounds.