[ He takes that as a "no" on the being sure that's not it, then. ]

... Seithr is an airbone substance with no smell, color or taste. It's not dangerous in small doses, but with high concentration it begins to corrupt living things, causing them to become more aggressive until they go completely feral. And they end up covered in black shit like that from head to toe.

[ He's not great at explaining things, especially ones that are considered obvious from his world's perspective, but this should shine some light on why it feels familiar to him. ]