This is the 12009th most frequent Russian word.
зеленое
"Зеленое" means "green" (neuter form).
In this sentence, 'зеленое' is used as an adjective agreeing in gender and case with the neuter singular noun 'яблоко' to describe the color of the apple.
У меня есть зеленое яблоко.
I have a green apple.
Here, 'зеленое' serves as a predicate adjective in a neutral form matching the pronoun 'что-то,' which refers to an unspecified object seen in the distance.
Я вижу что-то зеленое вдалеке.
I see something green in the distance.
In this example, 'зеленое' functions as a nominalized adjective acting as a noun to refer to a green item or object, where the actual noun is implied but omitted.