That is bizarre - not a speck of coraline in there.
Weird.
The only thing I can think of is that given good conditions, SPS will outperform coraline on Ca and Alk usage .. so maybe the coraline can't get a foothold. Add a couple urchins in there that eat coraline, maybe the urchins just have the upper hand on the coraline?
I have to say though ... "BAM!" on that showcase tank. Beautiful aquascaping.