There is white fuzz on my potting soil. Is it dangerous?

In the great majority of cases it is saprophytic fungus — a harmless organism feeding on decaying organic matter in the mix, not on your plant. It appears as white or cream fuzz on the surface, sometimes with a faint mushroom smell, and it is a sign that the surface is staying damp with little air movement.

Distinguish it from three things it is often confused with. Mineral salt deposits are crusty and gritty rather than fuzzy, and they scrape off as powder. Perlite is bright white, hard and roughly spherical, and it does not spread. Mealybugs look like small cottony blobs but are on the plant rather than the soil, move if disturbed, and leave sticky residue.

None of these need panic, but the fungus is telling you something worth acting on. Persistent surface damp usually means the pot is too large for the root system, the mix is too fine, drainage is poor, or the room has too little air movement. Fungus gnats thrive under exactly the same conditions and will follow.

Practical response: scrape off the visible growth and the top centimetre of medium, let the surface dry properly between waterings, improve air circulation, and water from below if the plant will tolerate it. A thin layer of coarse grit or bark as a top dressing keeps the surface drier and discourages both fungus and gnats.

If the fuzz is on the plant itself rather than the soil — grey furry patches on leaves or stems, tissue going soft beneath — that is different. Botrytis and powdery mildew do attack living tissue and need affected material removed, better ventilation and, if it spreads, an appropriate fungicide.