The green or brown algae is a different algae from coralline it will never turn purple like you're looking for. You can scrape that and anything else that isn't purple as you see fit. The best way to get coralline started is to get a rock that has some on it already and use a knife or another rock to scrape the coralline off and let it move around in the water column as little specks. Do this when your powerheada are on but your return pump is off. Biggest thing for coralline though is stable water parameters as it is a calcium based growth.