Algae Magnets come in two parts.
Both contain magnets, one half of set has coarse algae removal material on its face, and the other has a soft non-scratch material.
The coarse faced magnet is placed on the inside of the aquarium glass, with the soft faced magnet on the outside. Moving the outside one across the face of the glass drags the inner magnet with it, thereby removing the algae from the inside glass as it moves.