Green brittle stars would not be my first choice. They are increadibley opportunistic feeders. Anything that cannot get away or is caught "unaware" is a target.
If you like these kind of stars then I would suggest one of their smaller less obnoxious cousins. The black brittle or possibley a banded serpent star. There is still always the risk, but much less.
If you already have the green brittle, I would suggest supplimental feedng with raw marine meats like krill, siversides or shrimp. Keeping them fed can sometimes be all that is needed. Just be aware they do get quite large.
Cheers
Steve