"Snag" fired this volley in
:

Vinegar or just about any weak acid . How about that stuff they sell
at
the BigBox stores for cleaning mortar residue off new brickwork ?

Concrete is mostly sulfates, which don't dissolve easily in things that
won't adversely affect the boxes.

If the "stains" are mortar, they'll come loose with a mild acid. If
they're really Portland-based concrete, mechanical abrasion with water is
the quickest way.

Lloyd