Over a period of time, the black fibre scrubbing pads of the Stampin' Scrub will become "dirty". Here are directions for cleaning these pads...
Before first use: Rinse under tap water and pat dry.
To clean: Rinse under tap water and air dry.
You can clean the scrubbing pads while they remain in the tray, just be careful you don't put too much pressure on the hinges of the open tray as you may damage them.
You can also remove the scrubbing pads from the tray to clean them, but be gentle when you remove them as you don't want to break the tabs that hold the pads in place.
If you find that the scrubbing pads do not clean up well under tap water, which often happens when you have been using water-resistant or pigment inks, then use a small amount of dishwashing liquid to clean them. Always rinse well afterwards, until the water runs clear, and then air dry.