The adjustable cutting guide is made of aluminum, and while it is titanium-coated for added protection, it will be prone to oxydation over time. This is a conscious choice! Inox is not suited for the manufacturing process of this piece, and plastic will not be as strong. Aluminum being both extra rigid and lightweight can last perfectly with proper care. I recommend to wash it by hand as aluminum is sensitive to aggressive detergents and a machine wash will likely ruin the coating and affect its shine. If this will not impact its function in any way, it is a cosmetic issue that some consider important.
I personally do not care much that my guide becomes tarnished and gray instead of shiny over time, but you may see it as a quality issue, which is maybe understandable but also a bit unfair. Hand wash to keep its shine, or use natural recipes to remove the stains that may appear over time (prefer lemon juice or white vinegar over industrial metal polish, those being naturally safer options since the guide also touches ingredients).
Bottom line, even if you accidentally wash this piece in the dishwasher, the oxydation that will occur will not affect the guide’s properties, and if you so decide that the visual aspect is paramount, you may recover a proper shine with vigorous ad-hoc polishing and grandma’ secret natural recipe (e.g.: boiling a solution of organic acid such as cream of tartar, lemon juice or salt and vinegar with water, soaking for 5 to 10 minutes and scouring with a soap-filled pad). By my side, I simply decided to go without as I am content with the old patina effect that my guide eventually acquired.