Hard to tell from the pic, but it looks like a sea squirt or tunicate. Is there an opening or two in it? It is fine if so. It could also be a sponge...and is fine as well. Bristleworms are a good part of clean up crew. They will eat detritus and leftover food.I have taken maybe 2 out that grew large over the years. If you have an over abundance of them, then there is too much available nutrients{overfeeding}.