Clean up the green muck and you'll cause a House Crest (Hufflepuff) fragment to appear.
Now head to where you created the explosion. Use magic to assemble those supplies and form a street sweeping machine. Before you proceed, head back along the street to where there was a barrel with cleaning supplies. Place the ingredients in the cauldron at the end of Diagon Alley to produce an explosion. Stand on it and interact with the shopkeeper in the window to receive a House Crest (Slytherin) fragment. As you continue along the alley toward the back of the screen, note a patch of light in front of a shop to the left. Enter Diagon Alley and pass through the brick wall using magic.
