Answering here, noting the similar question you posted on your other thread Focusing and Chromatic Aberration Issues
I assume that the microscope still works in brightfield mode with a slide?
Is it as simple as brightness? The phone screen will not be as bright as the focused LED from the condenser. In the settings tab there is a “Auto gain and shutter speed” button which would adjust the camera to the dimmer image, if that is the problem.