It's hard to get it off. When the covers are old... if you get too aggressive... they just rip.
If you have a newer washing machine (without the agitator in the middle) you can put it on an extra long cycle, with hot water, and use Tide. Then... when it's done washing... set it out in the sun to dry. (inside out) It won't remove all the stains, but it will kill the mildew.
If it's in good shape... you can use a little superclean and a soft brush to loosen the mildew before the wash.
Just as a final FYI... I've thrown out most used covers I've gotten. They may look OK... but when you start to wash them... they disintegrate, and fall apart. The only ones I've been able to salvage were from skis that were stored inside most of the time.