Encryption at REST is supported for MySQL, Oracle, SQL, PostgreSQL, MariaDB and Aurora.
Encryption is done by AWS KMS service (Key Management Service).
Once the RDS instance is encrypted. The data stored at REST in the underlying storage is encrypted.
This is applicable for Automated, Read Replicas and Snapshots.
At present, Encryption of the existing database is not supported. To use Amazon RDS encryption for the existing database, you must first create a snapshot, make a copy of that snapshot and encrypt the copy.