My best guess as to what this is, is coralline algae. Here is an explanation of what this might be from Wikipedia: Coralline Algae.
Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of red, but some species can be purple, yellow, blue, white, or gray-green. Coralline algae play an important role in the ecology of coral reefs. Sea urchins, parrot fish, and limpets and chitons (both mollusks) feed on coralline algae. In the temperate Mediterranean Sea, coralline algae are the main builders of a typical algal reef, the Coralligène (“coralligenous”). Many are typically encrusting and rock-like, found in marine waters all over the world. Only one species lives in freshwater. Unattached specimens (maerl, rhodoliths) may form relatively smooth compact balls to warty or fruticose thalli.
If you have a better guess, let me know. Oh, this photo was taken on the Pacific side of Costa Rica.