Lean Ikea Krabb mirror against the wall (for decoration), need something to prevent them from slipping
Your cinder block holding a board provides an indication that the answer you seek does not have to be particularly elegant. You could place another board under the block, with the same cleat, positioned outside the base of the mirror. This would prevent the mirror from sliding away from the wall.

A less bulky alternative would be to keep the cleat, extend it wider than the mirror and attach it to the bottom of the vertical board with strong cord. This would allow the entire assembly to be collapsed when needed. It would be necessary to implement additional fastening to prevent the cleat from riding up the mirror. A slot in the cleat into which the mirror would seat would serve equally well in that respect.