Homotrema rubrum:

Species of encrusting foraminifer characterized by a distinct red wall. The colour is the result of the decomposition of chlorophyll pigments of endosymbionts. Globose, hemispherical, or encrusting with conical projections or erect branched forms are found. Large (up to 8 mm although they average 2-5 mm) calcareous attached foraminifera that is highly variable in form.

Grows on lower surfaces of shells, coral, and shoal or reed detritus, primarily on outer shoals surrounding the platform. Found in warm waters worldwide between 30 degrees north and south.