From the Super Mario Wiki
Strollin' Stus are creatures native to Isle Delfino that are found in the game Super Mario Sunshine. They are squat and pudgy, somewhat similar to Goombas, and have a bulbous nose, pink high-tucked spotted trousers, and a waddling gait. They are the most common variant of Stus encountered in the game. They will charge at Mario should he get too close. If sprayed with water, they flip over, allowing Mario to spray them away or walk into them to kick them away. Strollin' Stus can be defeated by using a Ground Pound, by having Yoshi eat them, or by spraying or kicking them into a wall, causing them to splatter and occasionally release Coins.
Strollin' Stus are known to stack themselves on top of one another with a Giant Stu on the bottom in a form known as a Stackin' Stu. If Mario knocks all the Strollin' Stus off the Giant Stu and jumps on the Giant Stu, he gets a Blue Coin and all the Strollin' Stus vanish.