confusing coralline

[Nihoa]

i dont really understand what coralline needs to flourish or how it behave one way in some parts of the tank and differently elsewhere. in our 33g reef, we have 40ish pounds on lr and on some pieces the coralline is doing fine while on other pieces it is frosting white and dying off. i gave some of the pieces a good brushing with a tooth brush to disperse spores and now all our hermits and snails are blooming with coralline on their shells and we have it growing on the glass but the die-off on some of the rocks continues. what gives? you would think that all of it would be doing poorly or at the very least you wouldnt have older stuff dying and new stuff spotting at the same time??

all our water parameters are normal although nitrates are a bit high (10pp). we have 50w of light (20 actinic, 30 life glo). i have been supplementing calcium with kent liquid calcium.

thoughts?