This is what can build up in your exhaust system, especially the fan, and can cause critical failure so its important to regularly maintain your exhaust system.
Here is how to clean the W150 Inline Fan that ships with the Mini 60 and Nova 24:
Here is more on that fan: W150 Inline Exhaust Fan Specs
Here is how to clean the CZ-TD550 Centrifugal Fan that ships with the Nova 35, 51, and 63:
Here is more on that fan: CZ-TD550 Exhaust Fan Specs