The white chalky substance that forms on the outside of such porous materials as cement and brick is most likely something called efflorescence.
This is a term, which in French means, "to flower out" and is almost always caused by some sort of water intrusion.
When water is absorbed into a non-organic porous material and then evaporates, this leaves behind the salts from the water. Materials such as brick and cement also contain salts, which start to be dissolved when the water is evaporated. While efflorescence itself will not pose a threat to your home, the cause of it may be the greater risk.
Water can be absorbed by brick or a similar material upwards through the soil or it can be from rainfall. This water absorption can be very destructive. However, some questions to consider are; what is the cause of the water intrusion? Is the substance efflorescence or mold? What are the risks of ignoring it? Can it be removed?
Please contact us and we would be happy to answer these questions.