I suppose the Tagalog reply to “Mabuhay” is “Kinagagalak kong makilala ka!”
And if this was Christmas in Hawaii, we’d say “Mele Kalikimaka” (thanks, Griswolds and Christmas Vacation).
Bubblebee Tuna!
But to be honest, we don't really use “mabuhay” in everyday conversations - for some weird reason we normally reserve it for greeting foreigners or during official government functions.
We prefer saying “kumusta? or “how are you?” with the expected response to be “mabuti” or “fine.”