BUT I would not put those in a reef tank. Seen them over run a tank. They do spread pretty quickly. I have few on a live rock i have been nuking it for few months. they always seem to come back from the dead. I got rid the rock.
You?re right they are hard to kill so good for someone new to corals and wants to put on an isolated rock. That?s what I did. Anyway if someone wants them let me know otherwise I will toss them