I noticed that my discus fish in my 46 gallon bow front tank has a disease that looks like a thread of slime with a white dot at the end. There are several of these things on each side of the body, mostly on the upper half. Also, in one small spot, it appeared as if the fish's body slime was coming off. I have had the fish since August, and have not introduced anything new to the tank since then. Yesterday, the fish appeared perfectly healthy, with none of these threads, and was actively swimming and eating. Today, it is not as active, but otherwise appears to be okay except for the threads.
The other fish, 3 koi swordtails and 7 cardinal tetras, don't have these strange threads on them. Ph is 7.4, Ammonia & Nitrite are 0, and Nitrate is around 10. I do weekly water changes of 50%. I feed the fish with flakes, pellets, and freeze dried Artemia. There are also a few zebra nerite snails added over a year ago.
What could this disease be? The closest thing I could think of was fungus, but it doesn't look much like fungus. Searching on multiple fish sites also failed to yield an answer.