The way I set up the green screen on my Apple Watch was,
1. On my iPhone , google green screen and then saved it to my phone album
2. On my iPhone, in Watch app I set a new face as Photos
3. I added the green screen as the only photo
4. Now on your watch , set your face as that and then you’re solid.
Btw, the clock can’t be removed to the best of my knowledge but you can mask it out after the green screen as well if you really wanted to with some key points. 😁