WHY DOES THIS HAPPEN
Yellowish-brown powdery crust on your plant's soil surface can develop as a result of Peat Mold. This is a type of mold that regular fungicides typically aren't effective against, however, it's not harmful to your plant - it's more just a potential eyesore. In especially advanced cases, though, it may interfere with your plant's drainage and ability to uptake water.
Peat mold (and mold in general) develops in moist, humid conditions such as a well-watered plant, particularly if the airflow around that plant isn't sufficient enough. This is another reason why it's important not to overwater your plant - as much as mold itself isn't usually harmful, overwatering can cause your plant a lot of damage.
This type of mold only develops in decaying organic material, like peat, which is where it gets its name. Therefore, you don't need to worry about the mold spreading onto your living plants.
SYMPTOMS OF PEAT MOLD
A crust on top of the soil, ranging in color from yellowy-orange to brownish, and often powdery in consistency
There may also be white mold on the soil surface too
Sometimes there may be a 'damp' odor surrounding the affected plant