Slimy Spring Galaxy

Overview

The Slimy Spring Galaxy is a location in Super Mario Galaxy 2, located in World 6.
It is a tiny water-themed galaxy.
 

Main Star Levels

The Deep Shell Well

Mario (or Luigi) starts of the level by jetting towards a cove in an underwater cliffside. 
It's a damp location with various rock formations covered in wet grass. On the left side of the cove is a Warp Pipe. This pipe leads to a room with three Green Koopa Shells and nine chests. Only three chests can be opened due to the shortage of shells, and some contain Goombas.
Further inside the cove is a Question Block, bat-like enemies called Swoopers and a waterslide.
The waterslide sends Mario (or Luigi) off  into the water below. Beware of the Mines that float atop the water, and follow the underground river until Mario (or Luigi) comes upon a Green Koopa Shell.
Grabbing the shell will let Mario move faster underwater. The river is soon replaced by an underwater tunnel that takes Mario (or Luigi)  farther down. 
Here are spiny enemies called Urchins, which should be avoided. Also dwelling here are plant-like enemies called Snoodles. These creatures will try to block the path for Mario (or Luigi). They can easily be dispatched with a Green Shell, or simply by light shone from the shell.  If Mario (or Luigi) keeps to the right of the tunnel when the stone pillar in the middle of the tunnel becomes visible, he will find a secret tunnel with a Comet Medal. 
The next area is a wider part of the tunnel. This place is teeming with Snoodles. 
Defeat them with light and continue before Mario (or Luigi) runs out of air. Farther ahead is an upward slope that Mario (or Luigi) must swim through while avoiding rolling Urchins. 
At the end of the slope there's a wall made of blocks, blocking the path. Use a Shell in order to damage the wall just enough to pass through. Just behind the wall are two Gringills (big eel-like enemies).
Dive before you reach them, and grab the coin further down to replenish air. 
There's a Launch Star at the end of the tunnel that will take Mario (or Luigi) to the next section
(There is a ledge below the Launch Star with a hidden 1-Up Mushroom).
 
A Checkpoint Flag greets Mario (or Luigi) at the very beginning of the next segment. 
A 1-up Mushroom is sitting on a ledge that can only be reached if Mario perform a Long Jump, followed by a Spin. Down the hole underneath the water are a number of walls with holes in them. There are rings in these holes that will boost Mario's speed. The clue is to get to the end  of the section before Mario runs out of air. Mario (or Luigi)  soon reaches another tunnel that's going upwards. There are several Urchin here, so be careful. When Mario (or Luigi) reaches the tunnel exit, he will be greeted by sunlight and a grassy field. 
On the left side of the field, a green shell can be found which must be used to open the treasure chest in the middle of the field. Once the chest has been opened, Toad will pop out, lament his own recklessness and hand over a Power Star.

The Chimp's Coin Challenge

Mario (or Luigi) receives a letter from The Chimp, who challenges him a final time. If Mario (or Luigi) manages to score 10,000 points by defeating enemies and collecting coins in a time span of 180 seconds, The Chimp will give Mario (or Luigi) a Power Star in reward. The layout of the mission is basically the same as the last, except that there are more Snoodles, more coins and no walls near the Boost Rings.

Bonus Stars

Green Prankster Comet

Both Prankster Comet stars are relatively easy to find.
 
Green Star #1
Run straight forward and use the waterslide down to the river. Dive, then pick up the Green Koopa Shell to move faster. Continue through the tunnel. When you get to the large area with multiple Snoodles, swim towards the second pillar (the one to the left) and you'll start to hear the sound of the star. The star is hidden behind the pillar. Grab the star to complete the level.
 
Green Star #2
Continue all the way to the first Launch Star, and then fall to find a secret ledge.
On the ledge is a 1-Up Mushroom as well as the second Green Star.