The sense of "be very green" that's most likely to occur to many people today is environmental sensitivity (or, eco-friendly, per previous answer). Of course the phrase can be used to say that a college has many trees and flowers, but that meaning is a ways down the list of possible meanings of "very green college", which include an inexperienced faculty or student body, a bilious group of people, and high frequency of green-colored buildings.
If you add 'campus' or 'grounds' after 'college', to indicate that you are talking about the outdoor appearance of the school, rather than about people, then confusion is less likely.
"Our college has much nature" and "There is much nature in our college" sound stilted and awkward. Perhaps refer to 'many natural features' instead of 'much nature'.