There's a pipe to the right of 6-1 that takes you to 6-B. You have 100 seconds to defeat one Fire Bro, two Hammer Bros, and two Boomerang Bros. Try to have the Fire Bro attack the others or pick them off one by one. After they're all gone, grab the Green Star.
Return to the main world map, then go southwest. You'll find a pipe that leads to the Sprixie House. The stamp is of the purple Sprixie Princess.