Diagnosis
That's undissolved detergent powder, and it happens a lot with high-efficiency top loaders. They use much less water, and if you're using powdered detergent or even too much liquid, it doesn't fully dissolve and gets redeposited on clothes.
How to Fix It
Stop using powder detergent. Switch to HE liquid detergent. If you insist on powder, dissolve it in a cup of warm water before adding it to the detergent dispenser or directly into the tub *before* adding clothes. Also, drastically reduce the amount you use—you likely need only 1-2 tablespoons, not the cup the box suggests.
If That Didn't Work
The problem might be the dispenser itself. On some cycles, the WT7400CW injects water into the detergent cup to flush it out. If the little water channel is clogged with hardened detergent, it won't work. Remove the detergent cup and rinse the compartment with hot water to clear any blockages.
Heads Up
This residue is often mistaken for mold or mildew. Run a cleaning cycle (with affresh or bleach) to rule out that you're seeing actual mold spots, but the powdery feel is a dead giveaway for detergent.