they aren't bad, usually. They are filter feeders that usually don't get the special care that they need, so they die. They need a lot of suspended particles in the water column to be able to get enough food for themselves. A lot of times they come in on live rock and usually just melt away, but there are a few species that will make it in the ree aquarium. I remember I had an orange one that came with my liverock that lasted for a little over a year. They are non photosynthetic, they just filter feed, but I wouldn't go out of my way to try and keep it alive. If it likes where its at it'll grow and live, if not, you'll get some extra ammonia for some small amount of time, judging by the size of it. But we are not 100% sure that its a sponge, it just looks like it might be. I would wait for some more opinions first, or take this to the invert or reef disscusion forums to see if anyone there can make a postive ID. That way you don't kill something for no reason.