They look like sponges to me. I had a whole bunch of similar-looking ones growing all over my nano before i took it apart. I believe these are the sponges Rob speaks of in the refugium show. In the last part they talk about the cryptic zones - a low flow, dark fuge - encouraging these sponges to grow. They are very good to have. They help reduce nitrates and free-floating crud. I don't think they will harm corals but i am not sure about that. From what i gathered poking around with mine, they are pretty delicate and i think the corals can beat them off.
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