It is normal to have some algae on the aquarium glass, gravel and ornaments.  You can purchase fish that thrive on it.  Sometimes algae will grow directly in the water, causing a green cloudy appearance.  Because it is too light to fall to the bottom or filter out, it is called suspended algae.  It is mostly caused by too much light.  Keeping your tank in a really bright room, or keeping the light on for too long can trigger this growth.  This condition can be treated by treating the water with a safe and mild chemical that gives weight to these algae particles.  That way, they are heavy enough to be cleaned out by the filter.  It's important to know that your filter pads will turn an odd color when you use these products.