I would just be guessing on top of my first guess, but if you've never noticed it before and it's been awhile since you ran your gen, then my first thought would be to check the battery at the terminals. If it's low, then I would tend to think that might be a low battery indicator. To test, I would charge it back up, stick it back in and see if that icon is still there. Like I said though.. just guessing here.
Someone will be along shortly and tell you exactly what it means. lol