Being a .mov shouldn't cause that issue on its own. Video files have wrappers and codecs. The ".mov" file name is just the wrapper. The codec is the way the file is encoded inside the wrapper, which you can see by opening the file in something like Quicktime Player and clicking on Window --> Show Movie Inspector.
Most files on phones are stored ...
The usual way to do this is with a speed effect. Create a cut around the one frame you want to render as a still. Your timeline should look like this:
motion-before | single frame | motion-after
You can then place a speed effect on the single frame, slow it down to 0% playback speed, and it will happily play forever if you want. Stretch single frame to ...
This should be a comment I guess but I don't have enough rep.
What kind of monitor are you using?
My iPhone records my 27" Retina 5k monitor just fine.
My guess is you have an LCD monitor and you are having polarizing issues (