After use, please don’t leave standing in water! If you do, or leave your brush in a wet and or cold environment, sometimes it could go mouldy. You need to let your toothbrush breathe and dry, as otherwise it collects moisture. It is a natural and untreated material after all, so it needs to be cared for as you would care for wood. Simply clean any mould off and dry the inside of your toothbrush holder. Preferably put the toothbrush in a place where it can stay dry and at room temperature and the problem should be solved.