Here is the full list of all 9 Chasing the Unseen achievements.
Reach the floating island at the center of the first level.
Catch all the critters in the cave level.
Catch all the critters in the mangrove level.
Catch all the critters in the octopus level.
Catch all the critters in the snake level.
Catch all the critters in the splash level.
Catch all the critters in the first level.
Unlock the secret level.
Reach the faraway floating island in the first level.