Is it still safe to ride the below tire, or should I replace it?
you can carefully prize the crack apart and see if it is just the rubber that is damaged or if it goes through to the casing. Your main focus should be where it extends to the sidewall as the tread area is protected by the thick rubber layer on a marathon plus.
If you conclude that it's only the outer rubber layer, i'd be happy running it as a rear tyre for shorter/local rides. I'd replace it if i were going on a longer or more remote journey.
If there is any sign that the actual tyre carcass is damaged then it is not safe to ride and should be replaced.