This is lichen’s growing on skulls. You can read the paper this image comes from here. Basically, it describes how in the 17th and 18th centuries, lichens growing on a human skull were believed to have healing properties. The lichen would have colonized the skull long after the person’s death. Lichens are extremely slow growing, so the skull would need to be exposed for a while and the whole process of colonization and growth would take decades, if not longer.
What I find most interesting about it is that it must mean that at this time, you could reliably expect to find human skulls lying on the ground in forests or other natural areas in order to collect the lichens. Maybe they weren’t exactly common, but obviously they were there. Probably the result of warfare, famine and epidemics. (Source)