Efflorescence may be an indication of of continuing moisture penetration through foundation walls and slabs or it could be residue remaining from construction and curing. Efflorescence is caused by sulfate crystallization. masonryinstitute.org publishes the following-
“…before sulfates can cause efflorescence the salts must be dissolved into solution by water. If no moisture reached the sulfates then they cannot be rendered into solution and migrate to the surface where the water will evaporate, leaving the sulfate salts on the surface to crystallize and become efflorescence. Attention must be given to preventing any soluble alkaline from being rendered into solution by water.”
Keeping downspouts and gutters clear of leaves and debris, to allow water to flow freely away from the foundation, and keeping landscaping at least 2-3 feet from the foundation will aid in preventing foundation moisture penetration.