Petractis

Petractis is a small genus of lichen-forming fungi of uncertain familial and ordinal placement in the Lecanoromycetes. These inconspicuous lichens grow as thin crusts embedded within rock surfaces, partnering with cyanobacteria to form barely visible films that are primarily detected by their small, star-shaped fruiting bodies. The genus contains two species that typically inhabit calcareous rocks, where their fruiting structures create distinctive pits in the stone surface after they decay....read more on Wikipedia.