Efflorescense, yes, but I'd say its from Quik-Set in the brick mortar and lime-and it appears the line of efforescense occours where the slab was "burned"etched when the brick were washed.
Salts in the mortar(some types of brick) migrate while damp and build up where the moisture phase changes to vapour, in this case where the damaged concrete surface meets the intact (less pervious) area.
Keep the brick dry, and it'll stop the movement of the Salts. Poor workmenship(clip joints) excerbates the probelm.
Next time, Leave your dead mortar inplace until your done washing down, just scrap it in place till done washing, when zero damage is required "y" another hose for a constant dribble of acid free water to dilute the wash down waste.
The salts might actually originate in the Crete, but the line shown is caused by acid damage--You could "burn" the whole surface so in the future the stain should be more even...
It appears the dampproofing/flashing was ommitted or covered by the concrete increasing the # of hours the masonry products are damp enough to allow the movement of the salts.
Also pouring stoops or porch slabs over earth or any continous sand, or finer fill with out a vapour barrier, or washed rock or a raised form floor keeps the slab wicking water from the ground.
The large width of the cracks has little to do with the white stuff, suggests very reinforcement used in the longer dimension of the slab
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