You could try to mirror the screen to determine if it is a display problem or perhaps a systemboard problem, before you open up the phone.
If the mirrored image is OK then more than likely it is a display problem or display connection problem, although it still could be a systemboard problem but this is less likely.
The phone will need to be opened and the display flex cable connection to the systemboard checked to ensure that it is a secure. If it appears to be OK then the display will have to be temporarily replaced and the phone checked to see if it is now OK.
Here’s a teardown video that may be of some help.
Replacement displays are available online. Just search for Galaxy J5 2015 LCD Digitizer Display Replacement to get results for suppliers.