What is this strange growth on my violet?

I've been growing violets for years now, and I've never had this happen. I don't even know what to call it. They are like white "lines" growing from a healthy leaf that eventually kills it and then kills the entire plant. Typically I use Doctor Optimara, but I can't figure out what this is! It kind of looks like roots.

This is the second violet from this seller that it's happened to (I won't name and shame because I'm probably doing something wrong).