Connect your RDS instance through master password.
mysql -h <host_url> -P 3306 -u <master_user_name> -p
Now in RDS instance (MySQL) shell prompt. You can create a new database and a new user.
mysql>CREATE USER 'newusername'@'%' IDENTIFIED BY 'password';
mysql> CREATE DATABASE newdatabase;
mysql> GRANT ALL PRIVILEGES on newdatabase.* To newusername@'%';
mysql> FLUSH PRIVILEGES;
mysql> SHOW databases;
mysql -h <host_url> -P 3306 -u <master_user_name> -p