Old English meos "moss," related to mos "bog," from Proto-Germanic *musan (cf. Old High German mios, Danish mos, German Moos), also in part from Old Norse mosi "moss, bog," and Medieval Latin mossa "moss," from the same Germanic source, from PIE *meus- "damp," with derivatives referring to swamps and swamp vegetation (cf. Latin muscus "moss," Lithuanian musai "mold, mildew," Old Church Slavonic muchu "moss").
All the Germanic languages have the word in both senses, which is natural because moss is the characteristic plant of boggy places. It is impossible to say which sense is original. Scott (1805) revived 17c. moss-trooper "freebooter infesting Scottish border marshes."