Graphis pulverulenta  

Thallus grey, white or usually pale yellowish-brown, uneven and cracked along the lirellae.
Teleomorph: ascomata elongate, curved, often stellate, to 10 mm long and 0.15–0.4 mm broad; discs visible, conspicuously white-pruinose on mature ascomata; true exciple black or partly white-pruinose. Ascospores 7- to 12- (to 14)-septate, 15–56 × 5–12 μm, the lumina mostly lentiform, rarely rounded or angular in section.
Chemistry: Thallus mostly K–, rarely K+ brown.