I have plating coralline right where a PH blows on a rock. Looks similar to a plating monti, but it's purple. My urchins love it! I wish they didn't because it's pretty cool looking.
The weird thing is, the frag wrack was from my old tank and it was covered in coralline, now it's not. It's making it difficult to out compete the cyano, and gha. I keep having to treat for them biweekly one then the other.
I have similar issues, my tank is 16 months old and started with dry rock. At close to the year mark I started to get coraline on the power heads and glass bottom and overflow but little on the rocks.
I just took it as long as I have it growing somewhere all is good lol