I think, after seeing a lot of pictures of "plume" on cigars -and pipe tobacco- that a lot of people actually have mold.
Not that there is anything inherently wrong with every type of mold. To be completely safe, you should know which type of mold it is, and then you might know if it is going to cause problems with your lungs, etc.
I'm guessing the risk is low in a pipe.
However, most "plume" I've seen is in the form of sugar crystals. I've found it most prominently on a box of Gawith's Full Virginia Flake and a Box of Hoggarth Bright CR Flake. Both with substantial age on them.
It looked nothing like the mold looking pictures I see pop up here from time to time.
I've also seen it on a number of cigars. A box of Bolivar Royal Coronas I had were coated in obvious sugar crystals. To me, it is extremely obvious. It looks just like sugar, glinting in the light.
This other stuff that some people say is also "plume" looks just like mold to me.
I'm not saying it's bad for you, because I have no idea about that. But I would put my money down that if we had it sent to a lab for analysis, that would come back as a type of mold.