that is basically why you are adding purple up , it is not a biological additive, you are not adding purple coralline algae, it is basically a calcium additive, supplying enough calcium in your tank for coral, and hence , purple Coraline algae to thrive. if you have a good amount of purple coraline algae, it is a good indicator that your calcium levels are right, or high enough, although by all means you should still test for it.
calcium , pH and Alk are all very closely related. if they are all ok , then you should add kalkwasser to maintain, or dose calcium if you have a voracious calcium -eaters- that will keep it steady. if one of them is way off , you need to fix it before the others get out of whack too