I want to say white lace sponge. I would actually remove it. But, that's me. I don't know if it would cause issues. I believe it could lower flow, depending on how it grows.
Usually I keep sponges, until they grow on places I don't want them.
Edit : good or bad is based on you. Sponges can compete for food and trace elements coral need. Honestly, I never seen a growth issue. Only with bad lights.
Is it hard and calcareous to touch? If so, you may have a vermetid snail outbreak. I know, they don’t look like snail snails, more like tube worms, but they’re a huge PIA to get rid of. I had a bunch ruin some internal pumps before.