White mold

You shall not worry dear grower, white mold on the soil is harmless, if you don’t like what you see, you could just use a spoon and pick it off or sprinkle some cinnamon on there, Though the mold on the soil is a sign of too much water and or bad airflow/circulation. Good luck dear grower

Is it living soil by chance? If so your good if not your probably watering to often and don't have enough air movement.

Creepy Steve pretty well hit the nail on the head........decaying organic matter that won't harm your plants. If it bothers you, break it up with a stick and let the top soil dry out a bit more between waterings, but it is harmless really. However, as Growing Grannie said, a picture would REALLY help just in case it is something else that could be detrimental.

more times than not it is "Saprophytic" fungi -- which means it only feeds off decaying organic material - real "organic" term and not the marketing term. So, it will not leech your nutes and won't cause any real problems. probably a woodchip or something else decaying in your substrate. if top layer of soil never dries, this is a more common occurence. you can kill it like shemshemet said, but that will also decimate any good microbes you have added or grew naturally. you would need to re-inoculate with any products previously used and then whatever ramp up time to re-grow... if late in life cycle may not be worth it. as long as it isn't seedlings, i'd let it go. a mature plant won't notice.

You know, even a simple photo would help us help you... but you don't have a diary AND you can't provide us with a photo? All I can say is good luck!

Find out more about it before taking action, it might be friendly. If it's pathogenic you could wipe out the microbiome with 1% Hydrogen Peroxide, or 7ppm of Chlorine Dioxide. Or, and then after letting it dissipate, innoculate the root zone with beneficials to at the very least prevent it returning.