They are a type of algae, and I don't think they will hurt anything. As far as i know, the only reasons to get rid of them is if you don't like the way they look or if they start to take over. I will warn you with my experience. I had several of them and they were getting quite big (about the size of a half dollar) I didn't mind them but my wife didn't like them so I decided to take them out. I tried to use a mechanical grabber and I couldn't get a good enough grip. I ended up popping it and it released spores into the tank and now i have quite a few more. The rest of them I removed with my fingers and that worked well, just don't squeeze hard.