Emminia Moss
It is always growing. Deeply seeded into the dermis, this moss forms a constantly regrowing protective layer over the skin of Host. For non-natural Hosts, it takes significant prolonged exposure for it to begin growing. A process referred to as Laying with the Dryads where after a few years it begins to grow.
Unfortunately, non-natural Hosts who contract the moss tend to have their lifespans shortened to less than three years, as the moss consumes them slowly, and near the end, in excruciating pain.
If measures are taken within the first couple of months, treatment is possible, easy, and at this point common medical knowledge.