I tried to flash the SuperSU v2.52 zip file with flashfire and it did restore /system/apps/SuperSU/SuperSU.apk with the correct icon except the problem is I noticed with Android 5.1 lollipop, you just can't replace a .apk in /system/app by moving it from /data/app as it will not even show up in the launcher. Not sure if this is a Lollipop thing or a LG thing as on previous Motorola phones running 4.4 Kitkat and below, replacing the .apk will usually work. I've cleared the app cache, deleted the files in /data/dalvik-cache/arm, /data/dalvik-cache/arm64 and /data/dalvik-cache/profiles without deleting the folders as I don't know if it's safe deleting the folders and also clearing the system /cache to see if it makes a difference. Didn't work.