what kind of hops did you use?
any idea how old your extract was?
Both hops and/or old poorly stored extract can lead to grassy flavors
Flavors reminiscent of chlorophyll and fresh cut grass occasionally occur and are most often linked to poorly stored ingredients. Poorly stored malt can pick up moisture and develop musty smells. Aldehydes can form in old malt and can contribute green grass flavors. Hops are another source of these green flavors. If the hops are poorly stored or not properly dried prior to storage, the chlorophyll compounds will become evident in the beer."