Saw this line in a paper I was looking at for other stuff, and thought of this thread.
Coral Reefs: Algal Ecosystems
"Many algae in reefs are weakly calcified (Udotea, Penicillus, dasycladaceans, Liagora, Galaxaura, squamariaceans, Padina, etc.), and under sporadic weak wave and current situations, a ‘‘dust’’ of aragonite crystals is often found coating virtually all algal surfaces."
Abundant aragonite dust produced on real reefs, would totally make sense if calcifying organisms equipped to make use of them.
(it's also a great paper on how powerful algae are in shaping what we think of as coral-dominated reefs.)