It is always a potential issue. The green more likely than others. Having said that I have had 3 VERY large greens, and 9 other largish stars with a cleaner shrimp for quite awhile and have no problems. But I feed very very heavily.
The important thing to remember is that these animals are following their instinct, not trying to annoy people. There is a risk with certain species of brittlestars - moreso than others - but it is never intentional. Feeding them and not allowing them to just scavenge is important, but it may not prevent predation.