As I understand it, by law, Claire Grady (the Deputy Secretary of Homeland Security) becomes the Acting Secretary of DHS in absence of Nielsen. But it appears that the President is circumventing that law, by appointing someone else to be the acting secretary. Did Ms Grady resign or refuse to perform this role?
(I fully understand that the President can nominate anyone he wishes, but that nomination must be confirmed by the Senate before the nominee has legal authority to perform the task.)
Additional information: 6 US Code 113(g) seems to have precedence over the oft-quoted Federal Vacancies Act (aka 5 US Code 33). The relevant section from the DHS Act states: "Notwithstanding chapter 33 of title 5, the Under Secretary for Management shall serve as the Acting Secretary if by reason of absence, disability, or vacancy in office, neither the Secretary nor Deputy Secretary is available to exercise the duties of the Office of the Secretary." (emphasis added)
UPDATED: Ms Grady has submitted her resignation as of today. So thje question, while interesting for a short period of time is now moot.