Indeed, that's the only word he used to describe its status when he answered a fan's question about the series on Twitter:
@DMC_Ryan Alive.— Phil Spencer (@XboxP3) August 4, 2016
Here's a recap of the public history of the Halo TV series. The show was first announced back in May 2013 alongside the reveal of the Xbox One console, with Steven Spielberg named as one of its executive producers. Originally, the show was going to be a part of Microsoft's Xbox Entertainment Studios, which was formed to create original TV and movie content for the Xbox One.
In 2014, Microsoft decided to shut down Xbox Entertainment Studios, but the Halo TV series continued to stay alive as a collaboration with the Showtime cable TV network. In 2015, Showtime president David Nevins was asked about the show's status and he replied, "It's still in development. Still in very active development." However, he also indicted there would be no new details revealed in the near future.
It's been a year since those comments from Nevins, and with Spencer's one-word status report this week, it would appear that the Halo TV series is still a thing. However, it's also apparent that we won't be getting any concrete info for some time.