Would you consider the Center for Disease Control a credible source of information?
CDC - Mold - General Information: Facts about Stachybotrys chartarum and Other Molds
Chances are, since the bottle is half empty, the water was contaminated with bacteria when you drank it. No, your mouth is not toxic, but the water is no longer sterile. Given time and a warm place (like behind your monitor), mold will grow. If the bottle was closed, it is unlikely that the source of the bacteria was airborne, so you probably are not breathing toxic mold bacteria.
But while we're on the subject of contaminated water... I recently researched a warning I'd received concerning bottled water left in a hot car... this WebMD article debunks that urban legend and provides more education about bottled water safety:
Bottled Water: FAQ on Safety and Purity