How do I delete from a mobile device and avoid deleting from My Cloud
What mobile device OS? What app are you using on your mobile device?
On my Android phone using the WD My Cloud app (edit to add: and the WD Photos app) photos removed from the phone are not removed from the My Cloud folder where the photo was backed up.
I have an iPhone 5s and my wife an iPhone 4s
My OS is the latest one 9.2.1