I assume you are trying to get to the spindle nut.
The cover is held on by 6 bolts that you removed. That leaves the plate and a gasket on the hub. Even a lot of rust wouldn't keep it from coming off with a little work.
Worst case, someone has covered both sides of the gasket with sealer that has turned to stone. Or worse put it together with some kind of sealer and no gasket
A thin putty knife is your best bet. Start the corner or side edge into the crack and tap with a hammer. Or if you can feel the edge of the cover just a bit proud of the hub anywhere around it, put something thin against that edge and hit outward.
