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it's a hard substance that encrusts on the rock and it is brown in color
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Peyssonelia is a brownish-colored red algae that forms tough crusts and is mistaken for a crustose coralline. But I don't think that the tiers of fan-shaped fronds are typical of it. Another brown calcified encrusting algae is Newhousia.
That exhausts my feeble library of references.