We have a section of waterline tile that has some kind of mineral deposit that has leached out of the natural stone above it and wont scrub off easily with just a stiff brush and water. I was thinking I'd work on it with a muriatic acid and water mixture, but I thought I'd post here first and see what kind of feedback I get. I certainly don't want to mess up the tile. I've attached a picture for reference. If I need to post more information about the tile itself, please let me know and I'll try to track down some details about the tile.