I agree with the comments to replace the fork or steerer ASAP. We had the fork fail on our tandem many years ago and examination afterwards showed a similar groove in the steerer although on ours it wasn't as smooth. Ours looked like a ball bearing may have fractured and only part of it was riding above the bearing race. When the steerer broke the front wheel separated from the frame and the bottom of the head tube smashed into the pavement. I ended up with scars on both hands and on my chin - definitely not a crash that I'd want to risk repeating. Fortunately my wife was uninjured (her half of the bike stayed up).

The tandem got a new steerer tube and had the bottom of the head tube rebuilt. It's still working ok 36 years after the crash.