I just bought three small potted cactuses. I have them sitting right next to each other on my desk. One of them is exhibiting very odd behavior. Every night, this strange white deposit forms on the outside of the pot the cactus is sitting in. Every day, I clean this deposit off, completely as far as I can tell. And the following night, the deposit regrows, as big or bigger than the day before.
The deposit is a white, powdery crust. It can be rubbed off with a finger, though complete removal seems to require a more vigorous scratching or scrubbing action.
The deposit forms some patterns as it grows. This pattern consists basically of nodules of deposit surrounded by halos of deposit. Some of the nodules are single spots, some of the nodules are lines. The longest line is about a centimeter long.
It is freaking people in my office out. Please advise.
-FrL-
This is going to be lime salts leeching out of the pot. It’s not mold unless it’s green.
You will see white deposits dripping from the mortar of new brickwork for a year or two. Washes off with the rain and reappears when there is enough damp to release it and not enough to dissolve it.
Okay, cool, how do I make it stop? 
-FrL-
While I agree that it might be salts leeching from the pot, mold is not generally green. Fungi are not photosynthetic, and thusly, are not necessarily green. I can think of several varieties that are white colored. Bacteria are also a possibility.
Wash the outside of the pot in a diluted bleach solution. If the spots come back, they are most likely a mineral deposit.
Re-pot the cactus. Use a plastic pot or one that has a glazed surface that won’t leak.
The pot it’s in is glazed ceramic, though…
-FrL-
Repot in an ordinary clay pot. Cacti are more prone to get overwatered and rot in plastic pots.