Yes, I would be concerned. That's your rear axle carrier subframe (or some other more accurate MB nomenclature), and it's a stress crack. The good news is that it's not fixed to the frame (it's unboltable), the bad news is it's a PITA to remove. I think your biggest problem is the cause of the crack, which is rust. It's probably manifesting itself elsewhere, so it's time to make some tough decisions about what you want to do with the car. As a stop-gap measure, I'd probably grind out the rust, drill a hole to stop the cracking, and pull out my MIG setup to weld a reinforcing plate over that area. Long-term, that subframe should be replaced with a junkyard item, along with all new rubber mounts.

YMMV