Moss and Lichen Growth May Deteriorate the Limestone

Lichens and mosses produce oxalic acid, which can destroy calcium-based stones. Since they are considered plants, they will also attract moisture, which can cause all kinds of problems with the stone, including spalling.

They can also cause the stone to break apart by causing pressure within the stone.

To remove moss try scraping or brushing as much as possible being careful not to damage the stone. After scraping a solution of bleach and water can be used to kill the remaining moss.