Crossword Genius

Reef-building organism (5)

Ross

I believe the answer is:

coral

(Other definitions for coral that I've seen before include "It forms reefs, generally in warm seas" , "Hard pink substance formed by skeletons of tiny sea creatures" , "Orangey-pink" , "Marine substance, often forming reefs" , "Basis of reefs" .)

I've seen this clue in The Washington Post and the USA Today.
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