I can not comment on how to get it out as I am unfamiliar with the installation.
Be very careful if you are prying back and forth on the cylinder. If the corrosion is severe enough it could rip a hole in the cylinder and cause a leak/explosion. I would highly recommend to empty the propane from the cylinder and keep the valve open when you are working the cylinder back and forth. If it is an older style valve the propane will escape at the valve by opening it. If it is a newer style valve, with the external threads, a regulator will have to be installed to allow the gas to flow.
Be safe.