Um. Algae grows in tanks, that is just what it does.
Did you cycle your tank before loading it with fish? What type of fish in the tank, what size tank is it, and how many fish of each kind are in it? The more bioload (feces, urea, etc) in the tank the quicker the algae grows. If you overfeed your tank with flake food it will make algae grow quicker. Flake food has high amounts of phosphates; an over abundance of phosphates will encourage algae growth. And generally getting your tank out of balance causes a lot of algae growth, usually.
Is your tank in high light most of the day?
Is there a light on the tank? If so do you leave it on at all times?
The more details you give the easier it will be for someone to answer your questions.
