Ambari shows that the regionserver component on one of the nodes is down. But actually that is not down, as i could see the regionserver process running on that node and processing requests. what could be the reason for this false alert. ambari agent is providing heartbeats in regular intervals too.
The alert isn't a stale alert - that's a completely separate alert which ensures that the alerts in the system are reporting in correctly.
Can you paste the error message of the alert? Is it moving back and forth between OK and CRITICAL, or staying CRITICAL all the time? Just because the RS is running doesn't mean it's working - there are times when the port threads have died, but the process is still in memory.