They can get invasive if let run wild. Some are kinda nice to look at though. We had a couple when we started our tank a year and a half ago and they multiplied to 100+ over the first year. They will sting anything they come in contact with but did not kill anything that we saw. If you try to pick it off you need to make sure you get it all because the smallest amount of foot left will just regrow a new anenome. As far as I know that don't spawn when threatened like aiptasia though. Once we had a large amount of them we put a Matted File Fish and it wiped them all out and then took care of the few aiptasia after that.
