To remove mold from the rubber seal on the water bottle spray the rubber seal with some straight vinegar and let it sit on the rubber seal for 2 to 4 hours.
The Vinegar will kill the mold and cause it to come off the rubber seal and then you can wipe it down with a wash cloth or paper towel.
If the mold still doesn't come off then you may need to soak the rubber seal in some vinegar for the 4 hours or even overnight for the mold to come off.
You could also use a new toothbrush to scrub the mold off the rubber seal after soaking it in Vinegar.