Sally, have you had a hail storm lately? That would cause widespread pock-marks to your semps. One thing to check is if there is damage on one leaf below another where hail could not reach it. That's one clue, check if pock-marks only occur where hail could fall. Having recently been a victim of hail I think it likely. From your picture it looks like damage out on the ends where exposed to hail. Also check if semps in one area of the garden are affected while another area is not cuz hail would affect all areas.
The millipedes pictured by Bev I seriously doubt are the issue. They may cause some damage but will remain down below underneath where it's moist and dark. If millipedes eat semps then my garden is history, cuz millipedes are extremely numerous in my soil.
