The walls in my shower are tiled with 6" tiles and have a decorative border consisting of small 1" tiles arranged in a dimond pattern. For quite a while water has somehow been seeping through the tiles/grout and have caused the sheetrock to start to mildew/mold. How can I repair this? I've tried sealing the grout but that didn't work. Should I remove part of the tiles and re-install them. If so, can they be re-used?

Thanks for your help.
Bill