Hydrochloric acid is a substance that merits a few initial words of warning, however.
It is a very corrosive substance that causes severe irritation to the eyes and exposed skin. Its vapors can also cause respiratory irritation. So it's a must to make sure you have adequate ventilation, wear rubber gloves, goggles and rain gear, and follow all the instructions and precautions on the packaging. Then you're just going to have to put some elbow grease into it. You scrub it vigorously to remove the plaster, then hose down the wall.