I never use purple up. It raises your calcium levels and causes calcium deposits to build up on the live rock and glass. But if you monitor your calcium levels to ensure you get to high, you can use it. It will help the coralline to overpower the green algae faster.
Purple up doses both calcium and Iodine, I reccomend against using it unless both are low, and you have the kits to test for them. if you can only test one or the other you could OD on the one your not testing for.