It's Caulperpa racemosa (yes, grape caulerpa).
Good or bad depends who you ask. Yes, it's invasive if allowed to become so. There's some mixed info in that other thread - it's not noxious to fish, many herbivores will eat it.
Yes it will crash if it's untended - if you want to keep it, keep it contained, and keep it pruned.
I had some in my refugium in my seahorse tank, crash, years ago. Woke up to see milky, cloudy water. No deaths/damage in my case but a big Caulperpa crash *can* foul a tank.
Jenn