My female fert Eris had this. In her case, it was a benign chordoma.
 
Our vet's advice at first was to leave it alone unless it started getting
visibly bigger or painful to the animal -- for example, if she often
caught it on something or started worrying at it.  Eris's tumor did keep
getting bigger, slowly, and once that was obvious we had it removed.  She
lost about 1.5 cm from the tip of her tail.  As surgery goes, it was a
piece of cake.
 
Here is some more information.
http://www.afip.org/ferrets/chordoma.html
 
Here are pictures of Eris's tail before and after surgery.
http://wrongcrowd.com/gallery/Util
 
Were it my ferret, I would not hesitate to get such a tumor removed
again.
 
Best,
Matt
[Posted in FML issue 4664]