Casa App uses an HD (hierarchical deterministic) key, which generates a new address with each transaction that's made.
When sending funds, leftover amounts in addresses get returned to the sending wallet in a new address. This is known as a "change" address.
Some hardware wallet devices, like Trezors, may ask you to confirm an additional "change" address when sending funds from Casa App.
For more information on how change addresses work and why they are built into the bitcoin protocol, see https://en.bitcoin.it/wiki/Change