Yes it is. Mine detached on my 99 a couple of years ago on BOTH sides (guess I never really looked up there).
Anyway, I noticed while at a grocery store parking lot waiting for my wife. Took me about 15 minutes since I did it without tools.
Obviously the accordion-shaped seal attaches in two places. The top slides back in a lip on the side of the moonroof glass, and the other on a lip along the track. If it hasn't come off completely, you can nudge it back in on both sides.
You can look at the one that hasn't come off and figure out where it's supposed to go.
I have chunky fingers, so it would have taken less time if I were at home with slim tools...anyway, it's pretty easy stuff.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car