Any pics? What color is it?
Either way mold or algae can only grow if your overwatering or using too much sugars (molasses, Carboload products, etc)
Chances are its from overwatering though. You should let your soil dry out between waterings, when the soil dries out it pulls much needed fresh oxygen to the root system.
After a while you will be able to know when to water just by looking at the plants, with soil you want to go as long as possible without watering but you dont want to go too long to the point that the plants wilt. You literally just want to water the day before the plant would dry out and wilt.
The best indication of how much water is still in the soil is to feel how heavy the pot/plant is after a watering, and right before a watering. It will be a LOT lighter when it needs to be watered. Also do the finger test. Stick your finger in the soil, if you feel moisture then wait longer to water.