Answer: Based on your description I suspect you are seeing a mold of some sort. This can be caused by lack of air circulation, over watering (these first two are the most common), lack of light, overcrowded seedlings, overfertilizing or fertilizing too soon, temperatures that are too warm or too cool, or in some cases, it just happens. Make sure you are starting with clean utensils and a relatively sterile soilless mix and avoid wetting the foliage when you water. You might try adjusting your growing conditions to try to keep it from spreading. If it is on the pot, you might try wiping it with a solution of one part ordinary household bleach and nine parts water (use this to clean your equipment, too.) If it spreads through the plants you might try watering them with chamomile tea in addition to the steps above.
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