You posted no pics or description of what soil and/or amendments or watering regimen, which makes it difficult to say for sure what is wrong and/or recomend steps you could take.

Algae grows in water/very moist conditions.Sounds to me like your soil is holding too much moisture. Read the "Container Soils - Water Movement and Retention XX" post below. it will help you understand plants and how they grow and how water acts in a enclosed environment, like a pot. You can make your own fast draining potting mix, or if you cant, or don't want to, at least switch to a "premium" potting mix that isn't mostly peat moss and drains well, and put in a bunch of extra pearlite, Perhaps a fan to help evaporate excess moisture.

A problem sometimes noted in commercial nursery production is the growth on the surface of pots of Liverwort. More common in low light, high humidity conditions. We used to peel it off and remove from the area. Most commonly found among pots getting too much water. Al