When mudding Greenrock, do you use just regular joint compound used with regular drywall or something else?

If you are mudding "cement board" in an area that is usually always moist. i.e. a tub surround or other bath area. You use thinset mortar. It bonds better with the cement board and will be waterproof for life. If you are using "greenboard" which is drywall with chemically treated paper to prevent molding, you just use regular joint compound.