With a little bit of work and prudence, yes, it can be cleaned. The best material and method to use is a bristle brush and muriatic acid. Although a very effective cleaning agent, muriatic acid is a very dangerous product to use, so it is imperative to wear rubber gloves with a long-sleeved shirt, goggles, and a mask. The bristle brush is necessary to clean the surface properly. Unfortunately, this results in splattering. Therefore the protective equipment is an absolute must. It is also important to make sure that the floor surrounding the wall is covered, and that the room is well ventilated.