Strollin' Stu is a Goomba-like creature that first appeared in Super Mario Sunshine as enemies. They are made of goop and can be defeated 2 ways. You either stomp on it or smash them into a wall with F.L.U.D.D. They make a reappearance in the game New Super Mario Sunshine as common enemies. You have to defeat them the same way. They also appear in Super Mario World: Wii.