Many microwave ovens are installed above the range, and sometimes (actually most times) the exhaust fan is not vented properly.
If you cook with oil and grease, the exhaust intakes usually have silver removable screens which catch most of the grease, and you remove them and clean them in the sink, but it does not catch all of the grease.
Most people do not even think about cleaning this exhaust because it is hidden, but you can have a grease fire in your house just like restaurants do. And they have their exhaust hoods cleaned regularly, or most do - the ones that don't do not stay in business because they have fires.
Flexible vinyl silver hose is NOT MEANT to be installed through walls, through floors, or be more than a few feet long depending on the application and sometimes the municipality, and IS NOT MEANT for microwave exhausts or rangehood exhausts.
Ductwork that can be cleaned internally is required, so if you have this coming from your rangehood or microwave oven it was INSTALLED IMPROPERLY and should be repaired, and should be repaired by a licensed and qualified contractor.