Diagnosis
The musty smell is mold/mildew growing in hidden damp areas. On the WT7400CW, the prime suspects are the rubber gasket around the top of the inner basket (the "bellows") and the fabric softener dispenser, which has a tiny hose that never fully drains.
How to Fix It
First, stop using liquid fabric softener—it's a major culprit. Clean the dispenser drawer thoroughly with hot water and a brush. Then, wipe the entire top rubber gasket with a 1:1 mix of water and distilled white vinegar, pulling the lip back to clean the hidden groove. Leave the lid open after every wash to let the tub dry out completely.
If That Didn't Work
Run a dedicated cleaning cycle, but use a washing machine cleaner tablet *and* two cups of bleach (not together, on separate cycles). The bleach cycle is more effective at killing the deep mildew. Also, check the small filter cap area (for OE errors) for stagnant water and debris.
Heads Up
The "Tub Clean" cycle alert on the display is just a timer reminder. It doesn't actually clean anything by itself. You must add a cleaning agent (bleach, affresh, etc.) for it to work.