The answer depends on whether you are on a free or Standard tier database.
Free Tier Databases
Free tier databases are not encrypted at rest.
Standard Tier Databases
Standard tier are encrypted at rest.
Your data is encrypted using the 256-bit Advanced Encryption Standard (AES-256), or better, with symmetric keys: that is, the same key is used to encrypt the data when it is stored, and to decrypt it when it is used.
These data keys are themselves encrypted using a key stored in a secure keystore, and changed regularly.
If you have questions, please feel free to write us at Gigalixir Support.