Is this a new thing, and is it all the time? Are your kitchen and bathrooms properly ventilated? Are you on a crawl space or basement?
Without knowing a lot more about your situation, here is a CDC doc that may help you: http://www.cdc.gov/mold/dampness_facts.htm
Mold is caused by moisture. If your seeing it near the bottom of the walls that is a common place.
This can be the result of not enough insulation in the walls resulting in cooler wall surface temps allowing condensation to develop. Much like the ice tea glass in the summer getting wet on the outside. The humidity is condensing on the surface because of the colder drink and ice on the inside.
In your case the humidity which is heavy sinks towards the floor area and the cooler wall temps allow that moisture to condensate causing the mold. You may not see this condensation as it can occur within the wall cavity as well. This is all determined by just how much insulation you have within the wall cavity.
Another reason for this to occur is cold air flow leaking up from below. If the rooms effected are over overhangs, or over a very cold air leaky basement that will also cause this issue and simply bypass the insulation all together.
Still another reason for this to occur is leaks from the outside of the house through the walls, windows or roof areas. Is there any water stains? This is a sure sign of water leakage. Perhaps a pipe leaking?
Once you fine the source of the moisture and stop it, the mold will no longer develop.You can continue to keep cleaning every day, but as long as that moisture is present, the mold will come back, worse the longer it takes to stop the dampness.
Another thing you need to check is the indoor humidity level. If your not venting showers or cooking with lots of boiling water, or if your heating your home with steam heat and allowing the air vents to continue to blow out steam, all of this will eventually cause condensate on walls that would not normally have issues if the humidity level in the home was low.
Are you getting condensation on the window glass? If you are and your not adding moisture to the air from cooking, bathing etc, And if your heating with gas, get the furnace checked. Burning gas produces a lot of natural moisture. And condensation on a window surface can be a sign that there is a leak on the flue system.
Could also be a pinhole leak of a waterpipe inside a wall. Copper pipe made in the 50's, 60's and 70's is reaching the end of it's useful life and will often spring pinhole leaks.