Seachem Algae Pad Uni

Seachem Algea Pads are scratch-resistant pads for glass and acrylic. The melamine foam uses micro-abrasive technology to easily remove algae from the sides of your aquarium. They are particularly effective at removing algae from scratches. They are chemically inert and do not use soaps or detergents. Their white color contrasts with dark algae, making it easy to see when areas are completely clean.

More about melamine:

Melamine foam is an incredibly effective abrasive cleaner. It has a microstructure as hard as glass, but because of its open-cell structure it remains flexible. It works on stains much like extremely fine sandpaper. In the aquarium, it easily removes algae by scraping with its extremely hard microscopic filaments.