Efflorescence isn't dangerous.

The white chalky stuff on basement walls is efflorescence — mineral salts left behind by water that moved through the concrete and evaporated. It is not mold. It is not dangerous to breathe. It will not eat your foundation.

What it does tell you is that water is moving through the wall at that spot, even if you've never seen the wall actually wet. So it's a signal, not a problem in itself.

You can wipe it off with a wire brush and a 50/50 vinegar solution. It will come back if the underlying water issue isn't addressed. If it does come back, that's when it's worth getting a real diagnosis.

Mold, by contrast, is fuzzy or splotchy and shows up in greens, blacks, or browns. Efflorescence is always white or off-white, always crystalline, always feels like chalk. Don't confuse them.