They sting friendly corals
Plus they reproduce really fast and grow even faster, and when they spread they sting whatever good corals they are growing up next to.
I have a big white aiptasia in my 10g mantis shrimp tank that I left in there to see how fast it would grow, when I feed the mantis frozen marine quisine I swish the food around in the tank so the juice flows around for the aiptasia to eat.
In about 1 1/2 weeks it has quadrupled in size. It's about 2" now. I will post pics when it gets huge. I am shooting for a half a foot if that is possible. LOL