Algae is a problem that the vast majority of fish tank owners have to tackle at one time or another. It thrives on the very things that the rest of your aquarium needs in order to grow - light, water and food. To much of any of these causes algae to appear, as does poor water quality.
There are many ways to tackle the dreaded green stuff, algae pads are particularly useful and inexpensive. They're great for wiping away algae from the glass and reaching all the nooks and crannies of the aquarium. They won't scratch the glass but scrub away algae quickly and effectively.
You may also want to try algae treatments in order to fight the problem along with test kits to keep an eye on water quality.