Moen cartridges are available. Their adds say "buy it for looks, buy it for life." Once you get it out you should be able to get a free one (more likely from a plumbing supply house, but a big box store should honor the guarantee). Anyway, take the escutcheon plate off so just the valve is exposed in the wall. Make sure the valve stem is pulled out when you thread in the puller. Twist the handle of the puller back and forth. Remember to take the clip out. You can try a little lime remover, but I'd got straight to heating it up. I'd use a propane or mapp gas torch and be careful!!!~ Take normal precautions. The heat will loosen it up; just be patient. It'll come, but it may take several attempts. Good luck!!