I agree with the ID, although not so sure about the second statement. I understood that their sting was very potent and will kill neighboring corals - I know they can be devastating in a larvae/fry tank.
They have covered every available rock space in my tank at times. They've been there from day 1- so 3 years ish. I researched heavily and was terrified by those same reports. I think it's logical that they would cause harm in a fry tank due to their size and impact on such small things, but I haven't seen any issues with my aquarium. I never had a fish show signs of irritation. My corals have also been fine. I tried many methods to remove them and really never succeeded. I have a Sea Hare now, there are a few animals like nudibranchs reported to consume them, but they are hard to find.
Aside from a breeding system- and generally while just starting out with hardier basic corals, I don't think it's anything to stress about. If you don't want them, you need to remove the entire branch system, which may be under the rock to start with. They grow out of the branch like a bunch of flowers and then float away in the medusa phase to live independently.