Recently we have been receiving some questions on the meaning of the 'grayed-out, Healthy' agent status icon (see picture below).
You will see this in the Monitoring space in the Computers view and the Discovered Inventory view and the Operations Manager ==> Agent ==> Agent Health State view. In short, the grayed-out icon means that the health service watcher on the RMS that is watching the health service on the monitored machine is not receiving heartbeats from the agent anymore. It had been receiving them previously (and it was reported as healthy), but now it is not. This also means that the management servers are no longer receiving any information from the agent at all.
To find out why this is happening, you need to check the Active Alerts for 'Health Service Heartbeat Failure', which generally indicates that the health service on the agent monitored computer is down but the monitored computer is still running and available on the network. If the agent monitored computer is down, you will see a 'Failed to Connect to Computer' alert for the monitored computer subsequent to the 'Health Service Heartbeat Failure' alert. The 'Failed to Connect to Computer' alert generally indicates either some sort of network connectivity issues or that the agent monitored computer is offline.