Black Sponge?

[Ace25]

I heard from a couple people many years ago that black sponges are bad to have in a tank. I never really thought about it much and I do have black sponge in my tank.. well yesterday when finishing up my algae scrubbing I notice a black sponge had overtaken the back side of my green slimer SPS coral..

So that is my question, are they bad, and is that the reason? They spread onto corals and kill them? This sponge is huge.. imagine a 10lb rock that is 75% covered in it, other 25% is my slimer on the rock itself. Toothbrush does nothing to get the sponge off the rock.

[Amphiprion]

They do voraciously encroach on corals and that is the main reason they are bad. Reducing in-tank particulates may not help, as some have been found to be photosynthetic, which also explains why they do so well.