Tony Raven wrote:

Baking soda or mild bleach then rinse well (its on the current
Camelback instructions). If its inside the drinking tube you need one
of their tube cleaning brushes and you can also fit your hand inside
the bag with a cleaning cloth to wipe it off.

Tony

Just read Tony's post......if you can get your hand INSIDE?? the bag
then there's no problem.

Just wipe it off. If it wont budge you may have a permanent stain which
is likely harmless.