There are a few Walleyes here, and some cogs to spin using the gears. Use the nearby gear to spin the red cogs so you can cross them, then walk along the orange cog to the first gem. It's on the other side of the wall. Cross to the other side using the red cogs, then spin the gear behind the Walleye to move the orange cogs. The one near where you started has the second gem. This also allows you to reach the green cogs on the other side of the wall. Spin the gear behind the Walleye to move the green cogs into place so that you can walk across the one near the purple pipe. Drop down and walk to the right, near the green cog next to the wall, to find and area with a hidden Gold Mushroom. Climb back up the ladder and cross the green cogs. Go to the other side of the wall by passing the Walleye, then cross using the red and orange cogs to reach the green cog, then cross to the other one. Dodge the three Walleyes to reach the purple pipe. Go through it and Blue Toad will give you the third gem. The plants have coins and a Mushroom. Return through the purple pipe, then climb the ladder to reach the Star.
Special Challenge: Find the hidden Gold Mushroom.