Answer: Healthy soil contains all kinds of micro-organisms plus recognizable animal life of several types. It's possible the soil contains worms (they're considered beneficial), especially since you're seeing what looks like worm castings. These little fellas will remain in the soil; you don't have to worry about them invading other parts of your home. I'd leave them alone, but if you're concerned, you can remove all the potting soil and replace it with fresh new potting soil. Dump the old soil in your garden where the beneficial worms can do some good.
Best wishes with your plants!
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