The screw adjuster tells it to take water from top of the tank with the skimmer or from the bottom of the device to act more of a normal intake. If your skimmer part is just floating I would imagine you have it unscrewed from the adjustment portion at the bottom.
I have one of these and really think it was kind of a waist of 20 bucks. It does remove any oily film on top of the water, but then again so would a paper towel. Other than that it doesn't do to much to aid in filtration, and actually hurts it a little if you don't have another filter to help pull the sludge from the bottom of the tank.