Make sure your strontium levels are high. Coralline algae is comprised largely of strontium. Usually it isn't low but surprisingly some people have lower than needed amounts. Also, aerating your RO
water before mixing your salt can help stabilize your calcium, alkalinity and pH levels if your not monitoring them. You'll need good water flow of course. Very high light levels can make growing it hard for some people, especially MH
. Not always, but it does prefer low bluer light in general. First step would be to check your strontium and alkalinity.
When you scrape, turn off your skimmer and remove your mechanical filter pads or sock for about half a day to keep the spores from getting removed. that way they can settle around much better. Good luck!