Crystals of the hardness minerals from the water, most likely.

Outdoors the ground is watered with soft rainwater, which will leach away minerals constantly, even if you watered with hard water in between rains.

Thanks for the reply.

I suppose that this crystals would not affect my planting, then?

The only time I have seen that kind of mineral buildup from my very hard water was in a pot of potting soil that was watered with the well water to the point of saturation and then left for seveal months while that water slowly evaporated. Normal watering with my very hard (mineeral laden) well water does not produce that even on plants that might have the normal nursery peat moss based potting soil, although I have seen some perlite or vermiculite float to that "soils" surface.
Are you sure that is not vermiculite or perlite?

It looks pretty clearly like crystal flowers growing on the very top surface of the soil. Depending on your water (which of course varies regionally) you can have all sorts of different salts in different amounts. Calcium and magnesium are the most common cations, carbonate and sulfate the most common anions. That's 4 possible salts right there. Also, I suppose fertilizer salts could contribute as well, but the vast majority is probably from your water.

Basically, 3/4 of the pot is peat moss, mixed with vermiculite, organic materials, perlite, epsom salt, bone meal, cottonseed meal, top soil, and etc.. Everything is pretty new. The perlite i have is kind of big in round shape. The white stuff on top of the soil is very tiny, looking like gritty sand

Epsom salt? I haven't heard of that as a potting mix ingredient. Depending on how much of that you used, it could be contributing also. Mag. sulfate. It can form cubic crystals, although other shapes can occur as well.

I did not put much though. A lot of articles suggest adding some epsom salt to the ground soil mixture. I assume that it is good too for pots

Most of the articles I have seen that suggest adding things such as Epsom Salts, gypsum, etc. are written by people being paid by the manufacturers to write that stuff so you will purchase those products. In the quantities usually suggested for adding Epsom Salts and/or gypsum to soils it would do little except waste your time, energy, and money. Adding a lot more can so unbalance the nutrients in your soil as to cause serious nutreint deficiency problems. Even coffee grounds and tea leaves can, in large enough quantities, create nutrient imbalances if nothing more is put onto the soil.

If someone makes their own soil-less mix (and millions of us do), the addition of gypsum is essential to the health of the plants. It's a great way to provide a couple of essential elements without putting the pH out of whack.

Not a fan of the Epsom salts in containers.

Anyway, your particular potting mix is one that cannot drain well. Those ingredients are fine for in-ground plantings, but not containers. The use of containers changes the entire workings of a soil/root system. Even the finest top soil can begin to act like the enemy within the confines of a container.

Since the drainage is going to be poor, you can expect that kind of crystallization to form on the soil surface. If you were to get rid of some of the peat moss, the vermiculite, bone meal, cottonseed meal, Epsom salts, and top soil you'd have something you could work with.