WHY DOES THIS HAPPEN
A white-colored crust forming at the surface of your plant's soil is most likely the result of Salt Buildup. This can be a consequence of overfertilizing in your plants, and / or may be also a sign of underwatering or poor drainage.
Some plants are more sensitive to salts than others, and buildup is more likely to occur during the summertime as your plant's water need is higher during the warmer months, meaning that salts are also uptaken at an increased rate.
If you tend to water using a method that doesn't sufficiently flush out salts from the soil then this can also increase salt buildup, so it's a good idea to thoroughly water your plants at least occasionally, and then be sure to empty out any excess water (and therefore any excess salts too!)
Note that the salts that build up in plant soil aren’t the same as the typical table salt that we eat, which is NaCl (Sodium Chloride).
A salt is essentially just a solid that can be dissolved in water. Fertilizers contain other types of salts like potassium chloride and ammonium nitrate, and some soil types naturally contain more salts than others. Alongside this, hard water from your tap can contain many salts which can be harsh on your plant. Therefore, each of these things can contribute to salt buildup in the soil.