That's a green "navigation light", a low-profile supplement to the main spot or flood light. Navigation lights are red, green, or orange, and are intended for immediate-area illumination (hopefully, without tipping off the enemy!).
To be real sneaky, use infrared lamps and glasses.
Some green targeting lasers are making an appearance, but I haven't seen one for a pistol yet.
Green would be superior for daylight use.
But you reach a point of diminishing returns... Anybody see the Alien Vs. Predator movie, where the visible beams of the green lasers, on the assault rifles, did a spectacular job of leading the bad guys to the shooters?
I was contemplating which tactical light to go with for the last few weeks. I went to the local shop to have my trijicon night sights installed by the dealer when I saw a GTL10 sitting all alone in the display case. I asked to look at it but decided I didn't want to spend the money on it that day.
Today was my birthday and my brother-in-law who was with me at the shop pulls out the box for the GTL10 and says, "here's something you can use."
I felt like the woman when the guy gets down on one knee with a slightly smaller box in his hand.
Finally, all these years of getting sox from the old lady, I get something I actually wanted!!