Head to the back of the map and find the goo. Wash it off and you'll find a guy near the 2 posts. Wash him off. Wait until he walks around and his back is pointing at the rope. Now talk to him so he flings you up. Now wash off the guy up there.

Shoot the little pink things at the piranha. He'll start flying around. Chase him and shoot him any time he stops for a moment. When he lands, shoot him in the mouth when it's open to inflate him all the way up. Stomp his belly when he's down. Three times is the charm! The shine is right near you on the ground.

Super Mario Sunshine Walkthrough