Hi,
not sure if the decals or original? The Cannondale name logo was up till 1991 at the top tube.
In 1991 also the new dropout design was introduced.
I would say this rather looks like like SM MTB frame before 1991, but a picture from the dropouts will help to identify the year.
Sure if the cantilever bosses on the rear are for u-brake only, it's definitely than older than 1990.
Colorwise Canndondale had SM500 and also SM800 MTBs in jade green, maybe this might fit, although the color of the frame at the picture looks rather matt. Is the paint job original?