The old stone was covered with mossy patches, giving it a lush green appearance.

mossy 是由名词 moss（苔藓）加上形容词后缀 -y 构成的形容词，表示"长满苔藓的"或"苔藓状的"。后缀 -y 常用来表示"具有...特征的"或"充满...的"。