Paint it to match the wall color. When a room has architectural detail built-in I try to avoid accent walls or multiple colors.

Yes. Paint it the wall color; it will disappear.

It is a picture rail. Some excellent advice here. https://www.houzz.co.uk/magazine/decorating-8-ways-to-incorporate-dado-and-picture-rails-stsetivw-vs~34712544

This is really helpful, thank you!

I've never seen a picture rail that low. That said, I would remove it, even though it's a giant PITA. I'd also paint that vent or speaker or whatever that white thing is the same color as the wall.