Fantail has white stuff behind gills!

Hey, all! I've found good help on this newsgroup before so I'm hoping
someone can offer assistance again. I currently have a 12 gallon
Eclipse
tank containing two goldfish, an oranda and a ryukin, both about 2-3"
long. The tank also contains one apple snail. They have all been
doing
well for about a year. Just did a 25% water change a week ago.
Yesterday, I noticed that the ryukin was hiding under a rock and had
some
whitish membranous looking stuff sticking out from under its left
operculum. I was a little concerned that it might be fungus, but when
I
came back to feed them, the white stuff was gone and the ryukin seemed
to
feed normally. I decided to keep an eye on him though. Today, I came
home from work and there was more of the whitish membranous looking
stuff
again sticking out from under the left operculum. I tried to take some
photos, but it doesn't really show up well. I then tried feeding the
fish, and the ryukin went after a flake but then seemed to "cough," and
in
the process of doing so, blew a bunch of whitish stringy/membranous
stuff
out from its gills. Now it LOOKS normal except I can still tell that
it's
not as active as it usually is. What the heck is this? The "coughing"
episode freaked me out. I'm putting it in a quarantine tank and
treating
for fungus, but honestly I'm not sure what it is. It's not like the
usual
cottony patches on the body that you see in books. Anyone have any
clues?
Does my oranda need prophylaxis? Any help is appreciated...