I scraped away the rust that was visible and sprayed it with pb blaster before I posted about it.Its probably not rusted seeing as the mounting stud looks rust free. They can get real stuck on there if they havent been removed in a long while. Grab the arm and firmly torque it up and down. Dont be afraid to mildly tap it with a hammer either. Using a lubricant like PB blaster could also help if its really stubborn.
Tried that and a hammer on the stud to no success.Did you try tapping and prying it yet?