On my equipped weapons hot bar, one weapon is highlighted green. I cannot unequip it, or scrap it. It can be moved into different slots in the hot bar.
And it can shift between weapons at a whim. This effect started on the starter pistol, but moved to an assault rifle on its own.

what is this effect and can it be turned off? It’s locked a low level assault rifle in my gun rotation and I can’t remove it. Playing on PS4. 

In your inventory, you can't unequip the weapon you are currently using. If you want to, you need to exit the inventory and change to a different weapon first.