The reed plates and reeds in nearly every model of Hohner piano harmonicas are made of brass, so water with some relatively mild detergent should be no problem at all for these reed plates. And in fact, mine sounds fine after going through the dishwasher.
The photo of your drip coffee maker suggests at least the top part is made from pot metal, also called white metal. Most of those drip coffee makers I have seen and touched in person have also been made of pot metal. Pot metal is about the cheapest metal in the world and has no standard of manufacture. Anything goes. There are numerous technical reasons why, but the use of pot metal is probably why exposure to a modern soap initiated an ongoing oxidation reaction in your coffee maker.
Beware of pot metal.