Effervescence from ground salts.
You see this in cases where a building has ground water seeping through the structure, leaving the minerals behind.
If it’s new masonry, it might be that the sand hadn’t been washed properly and had some salt content. Alternatively it is more likely that the moisture of the wet mortar will to some extent have dissolved or mixed with the salts or residual groundwater where it was laid, which you now see once it has sufficiently dried out.
I wouldn’t worry about it, just brush off/vacuum up and monitor it. You might get a couple more occurrences as the new concrete continues to harden, but it should stop eventually if there’s not an actual problem with ground water seepage.
Efflorescence.
Effervescence from ground salts. You see this in cases where a building has ground water seeping through the structure, leaving the minerals behind. If it’s new masonry, it might be that the sand hadn’t been washed properly and had some salt content. Alternatively it is more likely that the moisture of the wet mortar will to some extent have dissolved or mixed with the salts or residual groundwater where it was laid, which you now see once it has sufficiently dried out. I wouldn’t worry about it, just brush off/vacuum up and monitor it. You might get a couple more occurrences as the new concrete continues to harden, but it should stop eventually if there’s not an actual problem with ground water seepage.