Hello marc...
Algae grows naturally in a tank that hasn't been running long. The combination of new water, light, fish and plants will need to work for some time before the tank settles. Don't worry about it. It's a good food supplement for your fish. There are all kinds of small animals that live it that are good food for your fish. The plant also helps filter the tank water.
You can help control it and clear the water by doing large, weekly water changes. Add a stem plant called Anacharis. It grows just floating on the tank surface and produces a mild toxin that slows the growth of many kinds of algae. Keep the food to a minimum. A little a couple of times a week is plenty for adult fish, fry can be fed a little every other day.
Ramshorn snails will help remove the algae too. There are several natural ways of dealing with it. I wouldn't put any chemicals into the tank water, though.
B