Coccomyces dentatus

Coccomyces dentatus is a species of fungus in the family Rhytismataceae. A widespread species, particularly in temperate areas, it colonizes the dead fallen leaves of vascular plants, particularly oak and chestnut. The fungus apothecia, which form in the epidermal layer of the leaf host, resemble dark hexagonal spots scattered on a multi-colored mosaic pattern bounded by thin black lines. When mature, the apothecia open by triangular flaps to release spores. The anamorph form of C. dentatus is Tricladiopsis flagelliformis. Lookalike species can be distinguished by the shape of the apothecia, or by microscopic characteristics....read more on Wikipedia.