I've never seen something like that before. is it trailing off his skin at all? It could be just fungus that is attacking broken skin. it usually goes away with a clean environment. you can tell it's the fungus if it looks slightly hairy....like mould
I've never seen columnaris in person, but I've read that it doesn't show the fuzzy hair strands like the fungus does.
Both the fish I have right now are from petco, and don't have health problems. one was lethargic for the first few days, but soon perked up and has been a feisty little fighter every since. lol.