If you have stained the front of the slips with pointing mortar you can use a few techniques to remove the staining.
You can use an abrasive cloth which will remove light staining. This should not damage the face or colour of the brick.
You can use a brass abrasive wheel in a power tool to remove heavier staining. Do not use steel wire wheels as the steel will rust later on leaving orange stains on your bricks.
You can also use brick acid which is readily available from builders merchants. Using a diluted solution can quickly clean light staining and using a neat solution (carefully) can break down more stubborn staining. This can be used in combination with the previous two methods.
You must take note of the required PPE requirements of using i.e. power tools and corrosive liquids.