Diagnosis
That's degraded grease from the agitator's internal gearcase or drive coupler. A seal has failed, allowing lubricant to leak out during the wash cycle.
How to Fix It
You need to remove the agitator and inspect/replace the failed seal and grease.
1. Unplug the washer.
2. The agitator cap pops off. Underneath, there's a large bolt (sometimes a special spline drive bolt). Remove it.
3. Agitators can be stubborn. You may need an agitator removal tool or to carefully pry it up after the bolt is out.
4. Once removed, you'll see the top of the drive shaft and the gearcase. Clean all old, leaked grease from the area thoroughly with a degreaser.
5. Inspect the plastic gearcase for cracks. The common fix is to replace the entire agitator assembly (part #W11035621 often includes gears and fresh grease) or at minimum the seal kit. Repack with appliance-grade grease if replacing components.
If That Didn't Work
If the leak persists after a thorough seal replacement, the gearcase housing itself might be cracked. In that case, replacement of the entire agitator drive assembly is required.
Heads Up
That grease is nearly impossible to get out of clothes. Run an empty hot wash cycle with bleach or a washing machine cleaner after the repair to purge any residual grease from the tub before washing clothes again.