If you arrive at 6:30 am and have a six hour layover you’ll be leaving at around 12:30. This means you’ll have to leave Brussels at 11:30 am (or sooner). You will not find any “traditional Belgium food (Mussels) or maybe some Michelin Star restaurant/bistro type places” open before that time.
The best you can hope for is a nice place to have a coffee around the "Grote Markt" (Grand Place), and maybe a small snack. Those places should be open even on Easter Monday morning.
Shops will generally be closed.