There are a few factors that can start the growth of algae in your Biosphere:
1) The Nitrate Level is high, Nitrate encourages plant/ algae growth,. Check your plant and make sure that it is still a vibrant green. If the plant has died, it isn't filtering the nitrate as it should.
Another reason is the water in the tank has levels of chlorine. In this case, always remember that when you are doing a water change that you use Distilled Water, do not use regular tap water to fill your
tank when doing a water change, chlorine is very harmful to ADF. You can also use a method of a Dechlorinated solution. It can be bought at most pet stores or aquarium stores.
2) Too much natural sunlight (not an electrical light for the tank), Direct sunlight is algae's best friend, so try to move your tank in a darker area. You may also use a blanket to cover the tank for a week and it will kill algae.
Easy steps to eliminate algae in your tank:
1. Simply take a paper towel, napkin, or tissue and wipe the insides of the tank removing the algae. You might need a razor blade if it won't easily wipe off.
2. If the water looks "yellowish" it may be time for a water change, Remember to use distilled water when using Our Biospheres
3. Cover the tank with a dark blanket to eliminate all light to the tank for a week. This will remove the "food" for the algae.
4. Use your water extraction tool (turkey baster) to suck out any gunk that may have settled in the rocks. Uneaten food is also "food" for algae.