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It could be water spots. If you water from above the plants, the water on the leaves can dry and leave a dusty residue which is the minerals in the water.

If the spots are more fuzzy like cotton, it could be mealybugs. This article will help you with those: https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm

There are many reasons that a plant may lose leaves but the three most common are lack of water, lack of light and pests. Make sure that the plant is getting proper water and light and check it closely for pests.

This sounds like you have mealybugs. This article will help you:
https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm

It is likely due to the fact that the plants in the container have different water requirements. You may need to separate the plants that have brown spots from the ones that do not so that you can give them the correct water amounts.

This crust is normally a salt layer. It forms if you are using a rather hard water or if the fertilizer is a bit too much. I would gently scrape it off and dispose of it. Add a little new potting soil to the pot to replace it. If the plant seems to be doing fine, this is all you need to do. If the plant has looked a bit weak, you may want to use a distilled water or cut back on fertilizing.

It sounds like an issue with powdery mildew. The following articles should be of some help to you:
https://www.gardeningknowhow.com/problems/get-the-cure-for-powdery-mildew.htm
https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm