I have a reticulata that had the exact same thing. My book said that it was from damage when captured,not to worry clean water conditions would let it heal. 3 weeks later I notice the tail is getting smaller I think he lost half to one inch. Now I am panicing I added salt to tank with no effect over a couple of days. So I tried during a water change to add salt over rays tail and the next day the white was almost completely gone,I added one more tbl spoon and following day it was completely gone. Its been 3 or 4 days now no white on tail.If anyone else has experience with this I would like to know if the tail will grow back? How ever you fix this let me know I think this could be a common thing.