To do Alt + h m u on Mac in Excel, download and install Accelerator Keys, then press Option, followed by h, m, u.
Command: Unmerge Cells
This is how to do Alt + h m u on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + h m u splits a previously merged cell back into individual cells. The content stays in the upper-left cell after unmerging.