When building a stone wall or other stone projects, it is almost impossible to avoid getting mortar on the stones where you don’t want it. If you try cleaning off the mortar while it is still wet, it just smears on the stones. An easy solution is to let the mortar dry and, at the end of the project, clean the mortar off the wall with muriatic acid. You can apply these same techniques to most masonry projects involving cement products.
Muriatic acid is also called hydrochloric acid, spirits of salt, or acidum salis. You can buy muriatic acid at hardware stores and home centers. The acid must be mixed with water to reduce its strength so it does not damage the stone or other materials. But even when diluted, an acid-water solution must be used very carefully to prevent damage to materials and potential injury. Always use muriatic acid in a safe manner.
Source: The Spruce