The Casa App uses an HD (hierarchical deterministic) Bitcoin key, which generates a new address with each transaction that's made. Though we generally recommend not reusing addresses, any previous addresses that you've used for that keyset can be used again to receive funds, even though the Casa App is showing you a different deposit address.
Keep in mind that if you ever transfer funds to a new keyset in the Casa App - which you may do if, for example, you want to replace a hardware wallet device in the future - you should not send to that address, since it would then be part of your old keyset.
"What happens to my UXTOs if I transfer to a new keyset?"
Anytime you transfer to a new keyset (when rotating a key, or making a recovery transaction), funds from the different addresses in the old keyset (UXTOs) are swept and consolidated to a single address in the new keyset.
"Can I spend from a certain UXTO?"
Yes! In the send flow, you have the option to choose to bitcoin address you want to spend from. Simply tap "Funding Origin" at the top of the send window, then tap the address or addresses you want your transaction to spend from.