You cannot determine what column of data changed in a SharePoint list from Flow - you only know that a list item has been changed. This is the same behavior that exists when writing a SharePoint Designer workflow.
While this isn't what you want, I will offer it as a workaround. You can set an alert on your SharePoint list. The alert email shows a subset of the columns in the list but always shows what column was edited and the new value.
You can create a Flow that is looking for alert emails - you can filter on the subject which contains the list name.
When the Flow runs, you can create post a message to a Slack channel which will include the body of the email which includes an indication of the column that has been changed.
While this isn't exactly what you want, it may meet most of your requirements.
Here is a sample alert email:
