Personally, I cannot find a good source to explain how the green brittle star is different from the green serpent star. Most LFS
use the terms interchangeably. From my experience the smaller stars eat waste and leftover food bits. They are all opportunistic feeders that will eat a sick, dead, or sleeping fish if they come across it. The larger they get, the more predatory they become. The green ones seem to get the largest. There are stories about stars that lift up on their legs to create "traps" for swimming fish but I have never witnessed that behavior. I have witnessed a star that decimated my crew of inverts by reaching out and grabbing snails. They hide at the first sign of light, it is never an animal that you will be able to watch during the daylight hours.