I discovered that there is no 'stop' on the thru the wall skimmer like there is on an in-ground skimmer. So when the pump turns off, the weir can flop in/out of the opening, letting previously trapped stuff float back into the pool.
So I created my own stop. I used a window sash lock screwed into the face plate. It will turn in order to swivel out of the way so I can easily remove the weir for vacuuming.
It made noise as the plastic weir whacked against the stop. So I put a felt pad on the door. Now it is silent. I doubt it will last long, so I plan to come up with another solution to the noise. we'll see how it plays out.