This really got me wondering so I did some "Googleing" and here is what I found. Amboyna is the same wood as Narra, it is called amboyna when in the burl form, so I wonder if it happens on Narra too.
It is probably not mold or mildew, they require > 27% moisture content to grow, if your pens that that high of moisture you are going to have other problems. This is not unique to pens, I found razors, duck calls, etc all have the same problem reguardless of finish.
Here is a response that I found on a duck call fourm that made the most since:
The technical definition is called "blooming". It's a chemical reaction between the oil finish and the natural extractives in the wood. Exotics have all kinds of extractives, some water soluable, some that dissolve in solvents-(oil based finishes contain some mild solvents). I'm not 100% certain, but I believe an oxidization layer (that greyish/white matter you've been seeing) forms on the surface when the oils react in the presense of air. -That oil finish no matter how thick, does not prevent air from moving in and out of the wood. In a dry environment a gradient is formed that tends to pull moisture (remember our water soluable extractives and oils) toward the dry surface. Just like a vacuum. So there is a constant march of goodies from the innards of our call, to the outer surfaces. The good news is that it will eventually subside. The bad news is that no one knows when? So you just have to keep wiping it off. Just part of the baggage when using exotic species.
So it looks there may be some pieces of wood that will have less problems than others and that is why not everyone sees a problem. At some point it could stop but it might take quite awhile.
Good luck, Mike