I agree that sponges are good if not essential to have in a thriving system. The species of sponge will determine if it is invasive. In my experience the yellow and orange sponges never have been an issue. There is a black stinging sponge I have encountered though that will grow as a white mat under rocks but in the light it will turn black and form tubular columns. The black stinging sponge not only can be painful to handle but I have also seen it go to war with corals and win. There are so many species of sponges though that I find observation to be the best tool in determining if a particular sponge can harm corals.