my spray bottle has lived on the end of my ironing board for the past 40 years...it is always there, and always full. i have never had a problem with 'musty-water' maybe because i use it eveyday. i never put water into my iron...i hate sputtering/leaking irons so i use my spray bottle, i'd be lost if someone ever (put it away).for your situation i would clean the bottle with a bleach/soap/ water solution(to kill any mold) then rinse well and let dry then smell it to see if i got rid of the odor. if you are only going to use it occassionly i would make sure to empty it, give it a good baking soda/water rinse, let dry totally then put it away to keep it fresh for the next time, maybe put it away with the sprayer separate from the bottle so no moisture is trapped inside to mold.