a very small green or yellow plant without flowers that spreads over wet surfaces, rocks, trees, etc. 苔藓；地衣 
moss-covered walls 青苔覆盖的墙壁
Word OriginOld English mos ‘bog or moss’, of Germanic origin; related to Dutch mos and German Moos.
Idioms
a rolling stone gathers no moss
- (saying)
a person who moves from place to place, job to job, etc. does not have a lot of money, possessions or friends but is free from responsibilities 滚石不生苔，频迁不聚财