Julii Cory w/ gunk coming out of gill.
 

I have a sick 1 1/2 yr old Julii cory. It (he/she??) has what looks closest to a cottage cheesy looking stuff coming out of her right gill. The rest of the fish looks normal. I noticed this about 4 days ago. It has not grown or shrunk in that time. We've had this tank for 3 1/2 years (new to keeping fish) and this is the first problem we've had.
I have a 35 gal freshwater tank (1 TBSP salt per 10 gal), private well water to fill/water changes
Penguin 200 filter ("C" filters)
Filtered sunlight for about 2 hours a day
3 Mollys, 2 Julii Corys, 1 guppy, 4 zebra danios
API test kit:
nitrite: 0
nitrate: 5.0
ammonia: 0
pH: 8 (steady)
Water is 78 degrees
last water change was 2 1/2 weeks ago, 1/3 changed (took it down a little far)
Clean 1x a month, with 1/4 change usually
Feed: Tetra Flakes, AM and PM
All other fish appear fine. I have not set up a quarantine tank.
Any ideas or help greatly appreciated! I've scoured the net for a photo of what I see on the fish and I can't find anything like it! I can email a close up photo if you want to see it.

Here is a photo: Flickr: Jacalyn3's Photostream