Agreed,the apperence of green algae is a sign that your tank is doing well.But too much can cause serious problems...
Algae problems can be caused by a number of things,most likly poor light,or lights on for too long each day.Or even sunlight in the tank.
Another and more common cause is poor water quality.
Poor water quality can be caused by a million things ranging from overstocking,under/poor filteration,even not enough circulation..
So tell us about your setup in more detail(including water test readings and we'll try to figure this out...