DIAGNOSIS The musty smell is mold or mildew growing in residual water trapped in the pump filter housing or the rubber gasket. This model has a clean-out filter for that.
FIX Unplug the washer. Open the lower front service panel (it might just snap off or have a couple of screws). You'll see a small black hose with a cap on the end. Place a shallow pan or lots of towels underneath. Slowly unscrew the cap (not all the way) to let water drain out. Once drained, remove the cap completely and pull out the cylindrical filter inside. Clean it of any lint, coins, or hair. Wipe out the hose end and the housing. Reinsert the filter, screw the cap back on tightly, and close the panel.
IF THAT DIDN'T WORK The real smell culprit is often the rubber gasket around the top of the agitator or the underside of the lid. Wipe these down regularly with a 50/50 vinegar and water solution and leave the lid open between washes to let everything dry out.
HEADS UP That filter housing will have about a quart of water in it, so be ready. And screw the cap back on FIRMLY, or you'll have a leak next time you drain.