When you get that super gant mushroom, pass theat bridge with all the bricks. At the very end of the bridge, there is a row of grey bricks on the ground, grounpound it to get a secret entrance to the green star.
You actually don't need the Mega Mushroom to get this star. I just jumped off the bridge and steered Mario back into the level to get this star. --- 3DS FC: 2836-0188-5813 Name: Danny PM Me for Pokemon Friend Safari! (Steel: Klang, Magneton)