That is efflorescence, mineral salt left behind when water moves through masonry and evaporates. It is not mold and it is not harmful, but it proves water is passing through the wall rather than just condensing on it.
More detail
Efflorescence is the single most useful diagnostic sign in a basement, because it distinguishes the two problems that look alike. Here is what it proves. Water moving through block or poured concrete dissolves salts in the masonry. When that water reaches the interior face and evaporates, the salts stay behind as a white crystalline deposit. So efflorescence means liquid water has traveled through the wall from outside. Condensation, by contrast, forms on the surface and leaves no mineral deposit, because the water came from humid interior air rather than from within the masonry. That distinction changes the fix entirely. Condensation is a humidity problem: a dehumidifier, insulating cold surfaces, and controlling interior moisture sources solve it. Efflorescence is a water management problem, and no dehumidifier will address it. The simple test if you are unsure: tape a square of plastic sheeting tightly to the wall and leave it a couple of days. Moisture on the room side of the plastic is condensation. Moisture trapped between the plastic and the wall is water coming through the wall. A musty smell is a separate signal and usually means sustained elevated humidity somewhere, often in carpet, stored cardboard, or behind finished walls where nobody is looking. One thing to avoid: waterproof paint and sealer products applied over efflorescence. They fail because they are being asked to hold back water pressure from the wrong side of the wall, and they hide the evidence you would otherwise use to track the problem. Brushing off the deposit and repainting resets the diagnostic without fixing anything.