You were able to expand the VMFS datastore to use the remaining space and you saw the datastore capacity went to 6TB from 4TB? That alarm indicates the used space on the datastore exceeded the set threshold why by default is 75% I believe. Did you add more VMs to the datastore or do you have thin provisioned VMDKs that grew?
I would double check your used space is below the set threshold. If you want to check the threshold setting, go to the datastore and select Monitor | Alarm definitions and look for the alert name. If you expand the details section, it should tell you the triggers and what the thresholds are set to.
