The tail did have some puffy growth on it when I first saw it, unfortunately, I didn't take a picture at that point.. that was probably around 2-3am yesterday. I took it as fungus, dosed the tank and went to sleep.
By the time I woke up, any cottonly growth was gone.. after reading that columnaris often creates that "saddleback" damage on livebearers, I thought that would be it rather than fungus. That was when I took the pictures.
It looks like it has gotten much better today, the "saddleback" seems to be mostly gone.. at best I can only see a faint resemblance where it used to be. The caudal fin is still quite torn though, but clean water and time should fix that.
Have to wonder just how badly I misdiagnosed this though..