This is something that cannot be easily "fixed" and can be safely ignored.
As @WalterH has pointed out correctly, the problem resides in gromox starting up faster than the database does. However, this is not a problem, since gromox automatically reconnects until it has a valid connection.
Due to modularity and scalability of grommunio, the database not necessarily does have to be on the same system (multi-node installations, separate DB environments, etc.)
Therefore there is no hard dependency and what gromox is the maximum it can do (from /usr/lib/systemd/system/gromox-http.service):
After=mariadb.service mysql.service
