If there is mold inside the handle, I wouldn't use it anymore.
Their FAQ page: http://www.waterpik.com/oral-health/product-support/frequently-asked-questions/ says (under item #12): "You can use a generic mouthwash in your Water Flosser. ... If additives are added to the water in the reservoir, rinse the system with a reservoir full of clean water after each use. Failure to rinse the unit may result in loss of product performance and/or function. Growth of bacteria and/or molds could also be enhanced."
Re: Waterpik & mold
Their FAQ page: http://www.waterpik.com/oral-health/product-support/frequently-asked-questions/
says (under item #12): "You can use a generic mouthwash in your Water Flosser. ... If additives are added to the water in the reservoir, rinse the system with a reservoir full of clean water after each use. Failure to rinse the unit may result in loss of product performance and/or function. Growth of bacteria and/or molds could also be enhanced."