Answer: If you are fertilizing and watering from the bottom, then the white stuff would be a buildup of fertilizer. Water from the top until water runs out the bottom of the flat. This flushes the soil of any fertilizers that may have built up in the media. Another explanation may be that you are overwatering and mold is growing on the surface of the soil. Let the soil dry out somewhat between waterings. Stick your finger in the soil. You can feel the moisture. The soil doesn't have to be soaking wet. Anyway...remember that roots need oxygen and too much water will inhibit growth and eventually kill the seedlings!
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