To acknowledge all alarms you can do
$alarmMgr = Get-View AlarmManager
Get-VM | where {$_.ExtensionData.TriggeredAlarmState} | %{
$vm = $_
$vm.ExtensionData.TriggeredAlarmState | %{
$alarmMgr.AcknowledgeAlarm($_.Alarm,$vm.ExtensionData.MoRef)
}
}
The "Reset to green" option doesn't exist for all alarms, only for event-based alarms.
And the SetAlarmStatus method, that is behind this Reset to green option, is not a public API I'm afraid.