They actually are a good thing to have. They filter the water, minuit, but do filter. I figure that anything that filters my water is a good thing to keep around. Some can actually be quite attractive. No, they don't sting. I have corals around mine, it just grows around them. It has colonized a section of the tank, and is a beautiful purple. Started out being about the size of the dime, and four yrs later it is about the size of a baseball.