from what i understand, it's just 2 different types of coralline algae. there's also pink and other colors, each supposedly a different "species" of coralline algae.
<a href="http://www.garf.org/" target="_blank">http://www.garf.org/</a>
go here and look under:
how to/propagation/coralline algae. it tells you there that the different colors are different types.
it's got all sorts of good info there. <img src="graemlins//urrr.gif" border="0" alt="[urrr]" />