Mummy, unicorn's horn, and bezoars appealed to the imagination because of their unusual character, but even the most commonplace substances might develop supposedly medicinal virtues if they had unusual or gruesome associations. Usnea was a substance of this nature. It was moss; not ordinary moss, but moss scraped from the skull of a criminal who had been hung in chains. Usnea was an official drug in the pharmacopeia until the nineteenth century; it was carried by all apothecary shops, and the first edition of the Encyclopedia Britannica devoted a section to its curative properties. Usnea was present in the prescriptions of the best physicians over a period extending from the Middle Ages until well after the American Revolution.
Source: Howard W. Haggard, Devils, drugs, and doctors (1929).
image source: reddit
More info from Frances Larson, Severed: A History of Heads Lost and Heads Found
Paracelsus recommended the 'moss', or lichen, that grew on a dead man's skull for seizures and 'disorders of the head', and to bind wounds, on the basis that the 'vital spirit' released at death would be transferred from the skull into the lichen that started to grow on its surface. The fact that these skull-grown lichens were quite rare only increased the value of the cure. Skull moss seems to have been a particularly popular remedy in England and Ireland, perhaps because in these countries dead criminals were often left on public display until their flesh started to rot away and things began to grow on their bones. In 1694 it was reported that London druggists sold suitably mossy skulls for 8 to 11 shillings each, depending on the size and the amount of growth on them. . .
There were reports of people growing moss on stones and then spreading it onto the skulls of criminals, as a way of harvesting the tiny green plants for sale. In practice, apothecaries probably used anything that grew on skulls, and some things that did not grow on skulls, to maintain their supplies.