Why doesn't Heroku Connect automatically update Postgres schema when changes are made in Salesforce?
Connect doesn't automatically bring changes made to Fields or Objects in Salesforce, into the database as it is unable to determine what impact such a change might have on your running application. Instead, Connect will halt syncing for that Mapping and allow your other mappings to continue syncing as usual while you resolve the issue.
To resolve this, visit the edit page for the mapping in question. Doing so should reveal the newly changed Field(s) from Salesforce. At this point you should be able to check or uncheck the Sync checkbox next to any relevant fields, in order to bring your database schema in Connect into parity with that in Salesforce, and click Save. Doing so should clear the error and resume syncing.