It should just slide off. Might have become bonded with years worth of oil, crud, dirt, and who knows what else it was exposed to. Hopefully it wasn’t glued by someone in the past...seems unlikely but you never know. I would use a rubber mallet and try a few light strikes on or around it. Maybe applying penetrating oil on the receiver where the forearm inserts into the receiver face. Some heat might also expedite the process, heat gun would be best. Keep us posted.