The green brittlestar is a known predator in the while and certainly a threat to small fish and shrimp. Not all individuals will do this, but yes, it is a definitely risk. I have 3 large one's and no real issue, but I feed heavily and would be ok if it ate a fish or shrimp overall. The risk may not be high, but it is there.
A chocolate chip star is predatory on corals, anemones, clams, snails, etc and over time will absolutely eat something.