Well you need to consider that cleaner shrimp do, instinctively, try and hop on fish to clean them of parasites. That is normal behavior, and it doesn't change in tanks. These are not called cleaner shrimp because they clean the tank, which is a common misconception. They clean fish, and constantly try and advertise that service, which may make them look hungry. Nonetheless it is good to try and spot feed these guys when you feed the tank.
What kind of fish were these? If this is a young tank, with new fish, the odds of mortality are high (not predation by the shrimp but actual fish death). The mortality rate of fish in this hobby is actually very very high.