'X Files' The Musical -- David Duchovny & Gillian Anderson Reunite ... The Tune Is Out There

Scully and Mulder said to hell with the aliens, and instead poured their energy into rocking out -- well, soft rocking out -- but they goosed "X-Files" fans with some lip action.

Gillian Anderson made a surprise appearance during David Duchovny's concert at the Cutting Room ... and performed a version of the Neil Young classic, "Helpless."

David was playing to promote his debut album "Hell or Highwater" -- but he and Gillian will be getting back together in a more familiar setting soon ... FOX announced the "X-Files" reboot will premiere in January.

They did give fans of the show what they always wanted to see -- a couple kisses, but to be honest ... they were kinda BS friend smooches. They've got 7 months to get it right!