You may be a type of hornwort or liverwort (bog plants) or a true algae you have growing. It's usually indicative of overwatering. You can find some info on
bonsaiTALK Links if you search under "hornwort".
Remove the upper crust of soil and replace it with fresh soil. You should also consider reducing the frequency of watering to allow the soil to dry somewhat more between waterings.
Regards,
Matt