The translation of Dr. Holtz's sign language is "This is murder."
After gaining access to the security footage on Moon Base Alpha, Kira and Dash find the video clips of Doctor Holtz "accidentally" walking out onto the air lock without checking his space suit. They notice him make subtle signs directed to the camera inside the room. The signs don't seem to make any sense, but his body language seems to show that it was not his decision to walk out.
Dash and Kira were wondering whether Doctor Holtz's hand symbols were of any importance when Dash suddenly had a memory of his Great Granddad making similar symbols to his mother when he was younger. He recognized the gestures as the old forgotten American Sign Language.
When they find out that Doctor Holtz is using sign language, they quickly run his symbols through their computer's translator. When the computer finishes its scans, it confirms that Dr. Holtz was murdered. This scene is significant because it reveals that Dash was correct in thinking there was foul play involved with the death. It also thickens the plot and adds new mysteries such as who committed the murder and why.