It may have been Sponge, or some other invertebrate, but it now has the appearance of dead and decomposing life. The color comes from a fungus that assists with decomposition. Depending on what method is being used to start up this tank, more of that kind of dead and decomposing life will be seen as it goes along. There is a way to set up a reef aquarium where it doesn't happen like this.
Aloha,
Mark