It is a known risk with the green brittlestar.
If you don't believe it, see below. Yes, it was a live shrimp.
The green brittlestar is a known predator in the wild. While most of my stars do hide during the day, note that at least 2 of my greens are out in the open, and can reach well in excess of 20" tip to tip. these are huge stars, and not comparable in behavior to most other brittle/serpenstars in the hobby. However most do come out at night.
I happen to have 3 very large greens (and about 7 other brittles) with a cleaner shrimp and so far so good, but I keep them very well fed and realize it may happen some day. There is a risk there, so if you will be annoyed if it is eaten, then don't risk it. If you can shrug it off, then ok.
My cleaner shrimp does not clean fish, or at least the fish don't want it to. But that is what they would do in the wild. In the tank, I guess you can consider them part of the clean up crew as they are just as good as scavenging as cleaning fish. Unless you have very large fish, you may not see them cleaning the fish. I think in my case the shrimp is as large as the fish and that must freak them out.
The biggest risk is when the shrimp is introduced. This is true of fish as well. Brittlestars - especially greens - can "smell" this stress and the animal is in signficant danger at this time. make sure the brittlestar is exceedingly well fed when anything is introduced. Absolutely, positively, spot feed green brittlestars, though this may not eliminate the risk of it eating something.
