About two months ago I had polythene and sand laid on the ground below my floor void. There was a white substance in places on the ground and on the brick foundation walls beforehand. I'd assumed this was efflorescence. I've just noticed there is now (presumably) the same white substance appearing on the sand, near the perimeter below a south-facing bay window. Does this suggest that this is mould rather than efflorescence? The sand was damp when laid. If mould, what should I do?

Thanks.

Yep, efflorescence falling off the brick walls. Relieved.