It may be the water (too much calcium, etc.), but it could also be powdery mildew, which means you're not allowing the soil to dry at all between waterings, and that's never a good idea (at least the top layer, and more if the rest of the soil is soggy all the time). A good way to deal with soggy soil is to add lots of grit to it when potting - perlite, and/or very small aquarium gravel (not the painted kind, the glassy kind). As well, if you have PM, better air circulation around the plants helps, as does a less humid environment, but only if it's not at the expense of needed humidity. Don't forget that spraying foliage also wets the soil (and doesn't really help the humidity anyhow). You can try to scrape off the top soil layer and see how it goes.

If you water your plants from the bottom, this is just a natural occurrence. If that is the case, you should water from the top from time to time and flush out the buildup on top of the soil. Let the water run through thoroughly, and don't let the water set in the saucer, etc.

Powdery mildew, Luce, is a fungus disease that can only grow on plant surfaces and will not be found anywhere else. Depending upon the plant species and the particular mildew strain, powdery mildew can infest leaf tissues, stems, flowers, and even the fruit. Never potting soil.

The white crusty stuff is almost certainly mineral 'salts' (not necessarily sodium) left behind as water is evaporated from the surface of your potting medium. These can come from your water and/or fertilizer. It would be a good idea to break up that crust so that water can percolate down properly, and so that water vapor can continue to evaporate. You might want to leach your container periodically with bottled (or Britta-ed) water to help flush salts out.

I do my best not to over-water my plants, usuaully they only get a once a week watering. The mentions of a fungus I guess wouldn't be the case--whatever it is isn't really bothering the plant as far as I can see--no extra wilting leaves or anything like that.

I'll try to use my Britta water in place of regular tap water. Honestly, if I don't want to drink the tap then why would my plants? So I guess mineral build up could easily be the culprit--or maybe the plant food, which they did have recently.

Thanks for your help!

something else you could do to reduce the mineral salts is just submerge your entire pot in a tub of water or the sink. The mineral salts will dissolve and your problem will at least be reduced if not cured for a time. Of course a cactus prolly might take objection to this but i've done it with tropicals when i have neglected them in the past and it works great.