What does the red Arrow in multiplayer weapon select mean?
Did it look like the red N7 stripe in the screenshot below?
If so, it is an indication of the weapon's rarity (in this case, ultra rare). These backgrounds were only introduced in the latest patch.
If it looked like something else, my guess would be that it was a graphical glitch (especially if you're playing the PC version). I tried cycling through weapons with a geth engineer, and none of the geth weapons were visually distinguished from regular weapons.
Edit: Apparently other people are experiencing this too. See this post on BSN. It sounds like a bug to me.