Essentials will display an exclamation mark in the Diff column of the Commit Grid as a warning that the metadata that you have selected is not available in the source org. This can happen when you clone a work item or deployment and associate it with an org that does not contain one of the metadata components in the work item.
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If you do run into this situation, check the org and if it is not there, you can delete the offending metadata. If it IS actually there try refreshing the diff with the blue arrows icon above the Diff column (see screenshot below).

Happy Deploying!