This is the 505th most frequent Polish word.
zielonej
"Zielonej" is the genitive or dative singular form of "zielona," meaning "green" (feminine) in Polish.
Here, 'zielonej' is an adjective describing the color of the noun 'okładki' (cover), specifically agreeing with its feminine gender and genitive case.
Mam książkę w kolorze zielonej okładki.
I have a book with a green cover.
'Zielonej' here modifies 'trawy' (grass), and it is in the singular feminine genitive form to agree with 'trawy' in the genitive case.
Nigdy wcześniej nie widziałem tego zielonej trawy.
I had never seen that green grass before.
In this sentence, 'zielonej' describes 'doliny' (valley), and is in the feminine singular dative case, reflecting the grammatical rules of agreement and case usage in Polish.