In the slave's /etc/mysql/mysql.conf.d/mysqld.cnf I needed to comment out the 'bind-address' and 'mysqlx-bind-address' lines, stop the slave, restart mysqld and start the slave, again.
Having 'bind-address = 127.0.0.1' prevented connections from anywhere else.
Commenting it out allows access from all sources, not just localhost.
EC2 Security Group restricts access to port 3306 to just the master server's IP block.
Working great all around, now.
Thank you for your attention.
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