Seeing that, it looks like a dead pixel (really 2) in the display. I see them on laptops on a regular basis. It probably needs to be sent in for service.
Last summer I had an almost identical line on one of my TZT 3 16s that also appeared regardless of what else was being displayed. Mine was thinner. I sent the unit beck to Furuno over the winter and they replaced the display. You should get in touch with them.