Monte,

The impeller mounting screw is a standard machine bolt( clockwise tighten / counter clockwise loosen, righty tighty/lefty loosy)however, it does have a coating of loc tite on the threads and a rubber gasket seal under the head.

It will be difficult to remove, you should be using a nut driver or a socket wrench and socket instead of a crescent wrench, and you would also need to hold the shaft / impeller assembly to break the screw loose.

In some cases, you may have to break up the impeller and hold it with a pair of pliers, in which case I would reccomend this part,

Part number: AP3115180