How can I remove a dull spot in a quartz countertop caused by Magic Eraser?
Polishing stones is done by sanding, alternating from rough paper to very fine, from 50 to 3000 grit and water, and is finally polished with a compound paste or wax and a buffer/polisher.  I used to polish acrylic with up to 6000. Your eraser probably acted like a sand paper and the damage depends on its roughness.  Additional sanding may be warrant to smooth it off. You may try first car paint polishing compound sold in most places selling car spray paint and hand polish your spot.  If you have polishing tools it could be more effective and easier, but it all depend on the damage you actually made. If you can still see scratches or dullness after your initial polishing it may mean your eraser was too coarse and you may need to smooth it off and polish again. 

Be aware that stones are not impervious to stains. Sealers should be applied where cosmetics or food may come in contact.  You can also buy stone stains remover in most stone tiles retail stores, they are not very expensive and much less damaging than your eraser. Good luck.