Wikipedia
Rhodoliths are colorful, unattached, branching, crustose benthic marine red algae that resemble coral. Rhodolith beds create biogenic habitat for diverse benthic communities. Common rhodolith species include Lithophyllum margaritae, Lithothamnion muellerii, and Neogoniolithon trichotomum.
The rhodolithic growth habit has been attained by a number of unrelated coralline red algae, organisms that deposit calcium carbonate within their cell walls to form hard structures that closely resemble beds of coral. Unlike coral, rhodoliths do not attach themselves to the rocky seabed. Rather, they roll like tumbleweeds along the seafloor until they become too large in size to be mobilised by the prevailing wave and current regime. They may then become incorporated into a semi-continuous algal mat. While corals are animals that are both autotrophic (photosynthesize via their symbionts) or heterotrophic (filter plankton from the water for food), rhodoliths produce energy solely through photosynthesis (i.e. they can only grow and survive in the shallow photic zone of the ocean). Scientists believe rhodoliths have been present in the world's oceans since at least the Eocene epoch, some 55 million years ago.