as said tarps prior,
but in worse case everbuild make some mortar stain remover thats effective, flood it with water first.
worst case you use a wire brush, because its cementous it will etch a porus substrate by a few mil, so again flood it with water let it soak for a few minutes then just go over it with a wire brush gently and it will lift it up fine, or use a bit of wet and dry paper that stone masons use, theres ways and means of getting mono of any surface its choosing the right one.