Obviously this is for picking up 75 pieces of dust, on top of the other dust achievements. However, there are achievements for not picking up any dust:
You should be able to get all of these at the same time, and also save you the trouble of FINDING 75 specks of dust.
First, when you find a speck of dust on the ground, save your game in a new slot. Then pick up the dust. Make sure the achievement tracker ticks up when you pick it up.
Now reload the save. Voila! The dust has miraculously reappeared! Pick it up. Make sure the achievement tracker ticks up when you pick it up.
Now reload the save... the kicker is that if, once you pop the third achievement, you reload the save one more time, you'll have picked up no dust.
I suggest using the controller to do this, much faster than the on-screen load button. Put the cursor on the speck, then
... This should take 5-10 minutes for all 75 specks.
Now that you're clean (
), continue with the game and Messy World should pop at the end of the game. (I'm still testing... but the game requires two plays anyway in case it doesn't work.)