That's the problem with ready mixed bags of salad! I sympathise with the bug.
I hope you threw the salad away, they spray them with a weak insecticide to kill bugs but any which get stuck to the salad can cause severe stomach upsets due to streptococcal growths.
https://www.google.co.uk/searc...
It is a 'shieldbug' in the family Pentatomidae.
The colour (which changes to brown over winter) along with the pale edge to the pronotum, general shape, and yellow tip to the scutellum suggest Chlorochroa ligata.
http://bugguide.net/node/view/...
http://horticulture.oregonstat...
http://www.britishbugs.org.uk/...
The salad might have been grown in nearby states.
http://www.discoverlife.org/20...
Chlorochroa granulosa is similar .. the species epithet suggests a 'grainy' effect but it also has three white spots along the edge of the pronotum which your bug doesn't have.
http://bugguide.net/node/view/...
