Could be a type of coraline, if you take a piece out and dry it does it turn white? Or does it shrivel up like a macro algae? It's hard to tell from the picture which one, as I've seen both that look pretty similar. I'm pretty sure it's not anything to worry about.
AFA the xenia, sometimes it has a death wish and dies sometimes with no apparent reason, perhaps Charles Bronson has something to do with it I'm not sure.