WHY DOES THIS HAPPEN?
White spots can appear on your plant as mineral deposits or as a consequence of something called Edema. Mineral deposits are naturally occurring and are nothing to worry about. On the other hand, Edema can be a sign that something is wrong with your watering.
Edema is usually caused by too much water, especially when your plant has dried up too much and then is heavily watered - often also in combination with insufficient light. It may also be a sign that your plant needs to be repotted.
WHAT IS EDEMA?
Edema occurs when a plant's roots absorb water faster than the plant can use it, causing its internal cells to experience water pressure. This often occurs after a routine of watering in the evening or just before a significant drop in temperature. When this happens, the plant tries to get rid of the excess water through pores in the leaves. These cells can burst and die, then form plant 'blisters'. Eventually, the blisters erupt and ugly growths form in their place. As it's not fungal or bacterial, you don't have to worry about it spreading from plant to plant.
Small bumps or blisters on your plant - these can vary in color but are often white, yellowish or brown. White crystallized bumps tend to appear on the underside of leaves, whereas the more yellow-colored bumps can occur on top of the leaf. Brownish spots appear when the leaf tissue has started to dry out and die.
Your plant's leaves may start to curl
WHAT TO DO NOW
Luckily, it's relatively easy to treat edema - simply put, you just need to allow your plant's soil to dry up a bit before watering it again. This will give it a chance to recover and re-adjust.
In the future, try to water a bit less - always check how wet the soil is before giving more water, and never allow the plant to sit in water. You can find information about how wet the soil should be before giving more water to your plant under its individual 'Info' tab in Planta.
If your plant is in a large pot, it could be that it is a bit too big for your plant and is consequently not draining well enough. You may want to consider repotting it or placing it in a sunnier location to help it dry out faster (although be careful with this - some plants can be sensitive to harsh sunlight). In fact, increasing your plant's exposure to sunlight in general can be helpful to plants with edema. Again, just make sure that you don't burn them by placing any plants that are sensitive in too-harsh sunlight.
Finally, if you usually water at night / in the evening, try switching up your routine to water in the morning instead, once the sun has risen. This will help to prevent edema from recurring.