Crawl Space Repair - Efflorescence on Crawl Space Block Wall

That white dust you can see on the block wall is a result of efflorescence. Efflorescence is what happens when water travels through a concrete block like those seen here, dissolving salts contained within the block on its way through. As the water reaches the surface and evaporates, it leaves behind the dissolved minerals, which crystallize. To an extent, it's more of a nuisance than anything else, but over a long enough period of time, this will compromise the structure of the blocks.