It's just some algae that's started growing and is usually nothing to be concerned about.. but if you ARE concerned, try covering the soil with some white opaque plastic... algae forms where water and light meet.... to be honest, you may be overwatering your little one if the algae continues to grow and spread - and overwatering (the state of being constantly wet, NOT how much water you give at any one time) is the quickest way to kill your plants...
Good luck!