I had the same thing on the Pectoral fins of a couple of my Muskies. I've been using a product from Virbac Animal Health called Maracyn-Two (a broad spectrum Anti-Biotic) and it appears to be working, but has taken about a week to show some results (and there still is some remnant of the white spot there).
My local LFS said that if it looks like it has a Cauliflower like head that it's some bacterial Phenomenon that has been happening in tanks lately, and no one has been able to successfully treat, but that it also just takes care of itself (with no harm done to the fish).
He also hinted that it's some sort of Tuberculosis. I don't know what to make of this (I think he's full of ****, but I don't know, so...).