Should have included more information - multiple protrusions/areole (see additional photos posted), concave on top each holding unstructured mass of algae about 75-150µm diameter, (although many apparently eroded off, perhaps by my handling, leaving depressions). Unfortunately no apothecia. Thallus both K and C negative. Looks like identification not possible.
Don't think dark objects are parasite.
(Looking more closely at the specimen rock I’m not sure that it isn’t calcic. Don’t have HCl and vinegar was negative. Limestone and marble not common in this area.)