ok, here is my thinking on this; its probably papilloma (infectious wart). here is my thinking on it....
this condition is reported in jap fire bellies and is associated with virus particles in the cytoplasm of infected cells. now my book (amphibian medicine and captive husbandry, wright and whitaker, 2001) says its normal for them to be benign, small (1-2 mm), hypopigmented (lighter than surrounding skin) and likely to spontaneously regress (in 2-12 months)....but then it goes on to also say that in 30% of the cases it can be progressive, deforming and possibly fatal as well as sensitive to UV radiation and temperature dependent (i am assuming they are saying these conditions will make it worse?), it says they can be as large as 10mm or greater.
their descriptions are: "may be found anywhere on the body... raised.. nodular surface,are glistening... forms discrete, unbranched, foldlike projections.....may appear.. multilaminated as if the corneum has not been shed through repeated molts"
here is the history on it-;
i noticed the growth about a year and a half ago, once it got to about the size it is now i cut the majority of it off flush with the tail. it was very folded in cross section, at that time i did not do any lab work to look at it, i was curious to see if it would grow back. since then it has and now its getting to the point that its starting to deform the tail profile and wrap around, the newt isnt eating as well as he used to and doesnt like to swim much (he had bred between then and now though). he has difficulty shedding at the location because of this...i am tempted to cut the tail off ...will consult vet on this.