Clean compressed air to remove the loose stuff, soap and water on the cases unless it has some weird RFI coating that may be damaged and as others have said, once that's been done clean the boards off with Isopropyl or a suitable electrical cleaner.
Chemtronics have a range.
Might also want to consider a conformal coating for future.
That was my thoughts too but I wonder if there'd be some impact on cooling nevermind the risk of trapping some contaminates if the cleaning wasn't perfect.

Electronics can be washed in warm soapy water, rinsed and dried, even lightly baked to ensure all water is displaced. Maybe then an IPA bath, blow off and dried again before sealing
properly against environmental contaminants such as the soot.
Makes you wonder why it wasn't done at manufacture.......seems like it's not really "fit for purpose".
