Can i use joint compound to make a fake rockwall for reptile?

i want to make a fake rockwall out of styrofoam. I already have the piece cut out and shaped. i kno i have to put a grout on it but i was wondering if anyone made there rockwall with joint compound. also what would be a safe sealer to put over the wall....something that would not poision my chinese water dragon. this wall will see alot of humidity!!!