While traveling trough farm land the PCs come upon a merchant sitting on a wrecked wagon without a mule attached to it, hid face burrowed in his hands. He explains that he was robbed by petty goblins, unable to defend himself he had retreated. He asks the PCs to help him retrieve the mule before the goblins roast it, as a reward they may keep his goods. How hard can it be?
Foot Coral. It attatches to a foot like moss, almost gluing itself to an organism. It gets its food supply from the ground, and does not leech off the host in any way. It is not a malignant organism, and is entirely harmless. But it only grows on human feet, and even then, never on the left foot. An organism that it grows on can feel touch through the coral, but not pain. Removal of the coral is painless, but why would you want to?
Texturally, if fells like a dry, non slimy amoeba.