Just a thought: Muriatic acid is not a cleaner. It does combine with some minerals that may be on the surface of the rock, making those minerals disappear. And, that might loosen some of the biofilm on the rock so it brushes off. But, if this works, the rock must be reacting to the acid, meaning it will slowly react to the acidity of the tank water, raising the hardness of the tank water. If that is not a problem for you then the acid wash is a good method for you. If it is a problem, that's a different story.