I get 'DE:AD:C0:DE:CA:FE' on my network

You have not been hacked.

Since version 1.10.9, Fing has added a 'DHCP server Finder' inside the Fingbox. This discovers every active DHCP server inside the network by sending a fake request with the special MAC address 'DE:AD:C0:DE:CA:FE', which may be recorded as a new device by your router.

Knowing the list of active DHCP servers helps to troubleshoot specific issues, and is considered standard practice for performing this type of test.