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Halimdea incrassata spp. is a green algae of which there are several other species in bermuda.  Typically found associated with seagrass beds or sand flats, it anchors to the bottom with that lump or holdfast you see in the image.  It has a calcium carbonate skeleton with the plant tissue living on top of its skeleton.  The actual mineral formed by the calcium carbonite (aragonite) is the same mineral as that formed by corals.