Diagnosis
That's dried detergent residue, usually from using the wrong type of tablet or powder in combination with hard water. The DUT25401X has a built-in water softener, but it needs to be set up and regenerated.
How to Fix It
First, run a hot wash cycle with the dishwasher empty but with a cup of white vinegar placed upright on the top rack. This helps dissolve existing residue. Then, check and set the water hardness level. Press and hold the "Delay Start" and "Extra Dry" buttons together for a few seconds to enter the settings menu. Use the "+/-" buttons to set the level to match your local water hardness (you can find this from your water supplier).
If That Didn't Work
If the residue persists, you might need to refill the integrated salt compartment. It's in the bottom of the tub, under a large screw cap. Use only dishwasher regeneration salt, never table salt.
Heads Up
Avoid using "all-in-one" tablets that contain salt and rinse aid if you have very hard water. The machine's own softener system works better when you use separate salt and a simpler detergent.