Answer: It's most likely not bad, please don't worry. Based on your description, I can think of two probable causes.
If it is a crust forming on the soil surface (and maybe on the outside of the pots as well, if they are terra cotta pots) then it is most likely excess salts build-up (from fertilizer and/or minerals in the water.) This can be simply scraped it off and discarded.
To prevent this from happening, periodically flush the soil by watering the plants quite heavily, let the excess run out and immediately water again. Then let the pots drain thoroughly. This leaches the excess out of the soil. Also take care not to overfertilize your plants. You can also use rain water rather than tap water if you have a lot of minerals in your water.
If there are white stringy growths winding through the potting mix, that would be a fungus working as part of the normal process of decaying organic matter. It is harmless to the plants and can be left alone. Once the organic matter is depleted it will disappear on its own. There is nothing you would need to do for this case.
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