A factory reset reboots the entire system. It deletes everything in the system by reformatting the hard drive. Hard reset is a form of factory reset, in that it only reboots the hardware on the phone. A hard reset is used to put as many settings back to their original state as possible.
I have an s6 active and I drop my phone, multiple times a day. I have also had a screen flickering problem, only about a half inch on the bottom of my screen flickers, mainly when I use Netflix. Turning down the brightness on my screen makes it stop flickering completely. Hope this helps.