Often a green apple flavor comes from naturally occurring acetaldehyde. Its common for yeast to produce this during fermentation, but is then cleaned up by the yeast before bottling time.
Is it already bottled or are you sampling readings?
If its already bottled it could have been bottled too early before it had a chance to clean itself up, and american lager strains do tend to throw this flavor out often, but if you are claiming and sweetness could be a sign of incomplete fermentation.
Either way I would recommend make sure youre pitching enough yeast, oxegenating the wort, use some yeast nutrient, and try to keep temps stable and you should be fine.