That depends upon just how much grease and dust we are dealing with here. Normally I would say, wash it off with soap and water, but if there is a large amount involved, you might want to first scrape off as much as you can with a spatula or other scraping tool, and then wash with soap and water. Ideally, dry afterwards with paper towels. You don't want to leave the walls wet, they could get moldy. Paper towels will also pick up any residue of grease and dust that were not washed off.