I like to open them up and clean them out By hand. This usually helps me to see the condition and see if there is something that really needs to be looked at especially if it is a used melodica. If you make it a practice to brush your teeth before you play, they stay pretty clean. I use a water pick, beilieve it or not, even with a good brushing the water pick loosens up material that is caught in between your teeth that can go inside your melodica.
Hope this help