Answer: Though unattractive, the mushrooms are harmless. Just pick them when they appear and throw them away. They are growing because there is some decomposing organic matter beneath the soil surface. Rotting wood (perhaps from the inside of the barrel?) or decomposing potting soil are perfect nourishment for mushrooms. As long as you don't consume the mushrooms, they shouldn't cause problems for you. They won't taint your peppers. Rinse the peppers off before eating and they will be fine.
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