What causes the leaves of houseplants to turn black, wilt and falloff?
There are a few things that can cause this. The first maybe that the plant is underwatered. The second could be a fungus on the leaves. The third could be root rot, which is commonly caused by overwatering, but can also be caused by a fungus.
I would check that you are watering correctly for the plant you own and also treat the leaves and soil with a fungicide. That will cover all your bases.
this is a indoor garden
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
I have a medium house plant that has started to lose a lot of leaves.
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.
I have white fuzzy stuff on some of my house plants that "squishes" like a live bacteria.
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
What causes brown spots on tips of some plants but not others that are planted together?
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.
I noticed a white powder or crust on top of the dirt in some indoor clay pots. Can it be turned under into the dirt? The dirt is potting soil, so should be good.
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.
I have white dandruff-like specs on one of my plants. I know this a common disease, but maybe you can give me the cure.
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