Laitance and efflorescence are really two completely different things, but they are both white, powdery substances, which can be confusing.
In short, laitance is formed when there’s too much water in the concrete mix, while efflorescence is the deposit on masonry surfaces caused by soluble salts in the units or in the mortar.
Laitance can be avoided by controlling the amount of water in the concrete mix. Efflorescence can be prevented, or at least minimized, by selecting materials free of harmful salts and by preventing water from penetrating the masonry. This may be accomplished by the use of solid and tight mortar joints, capped walls, effective flashing, and adequate weather protection of the masonry during construction.
Both laitance and efflorescence can present both cosmetic AND structural problems.