Arthonia cohabitans

Arthonia cohabitans is a species of lichenicolous (lichen-dwelling) fungus in the family Arthoniaceae. The fungus forms minute brown-black flecks that develop beneath the bark surface but soon break through and become visible, typically measuring less than 0.2 mm across or forming elongated structures up to about 0.3–0.4 mm long. It produces small ascospores that start out colourless and smooth but later become brown and finely textured, with the upper cell larger than the lower....read more on Wikipedia.