Given enough light and nutrients, algae will grow on almost every surface in an aquarium. Although it may make the aquarium look dirty, it is not usually dangerous or harmful to fish, but most aquarists wage constant war against it anyway. To most, even a luitlle algae means the aquarium is not crystal clean. Although I always use any medication with caution, algae removers do exist, Aquarium Pharmaceuticals Algae Remover Tablets and Algae Destroyer Liquid are two of these. If you have live plants in the aquarium, always use any algae remover with extreme caution since algae is also a live plant and algae eliminators can cause difficulties to live plants as well.
Ammonia is can have either organic or inorganic sources.
In the aquatic world of your aquarium, most of the ammonia is produced organically. Fish live and respire, giving off ammonia through the gills as they accept oxygen. Fresh water fish also urinate constantly, inorder to eliminate the water forcing its way in through simple osmosis. As well as this excess water, ammonia and urea is eliminated. Decay bacteria work constantly to reduce wastes to the most simple compounds.