See this sponge:
This spreads rapidly in my system and I would say it's not entirely harmless. It can choke out zoa colonies and most recently when I broke down my tank for a move, this is what happened:
I wanted to removed all the corals a week prior getting the fish out, I started pulling out some of the rock to get to all the corals. There were so much of these sponges that pieces started coming off rocks when moved and were suspended in the water column. Unknowingly, I started to turkey baste some the rocks after all the corals were extracted and this really suspended a lot of these sponges. Next thing you know after the water clears up from all the disturbance, 1/2 my fish population was lying on the bottom breathing rapidly and 1/4 of them died. It was all the toxin from the sponges I'm guessing.