[954Stealth]

It is hard to say how it will affect your credit score without knowing what else is on your report.
That said, an open card with a zero balance is better than a closed account.
Utilization accounts for something like 30% of your credit score. They look at each credit account individually and then all the credit accounts together as part of the credit mix algorithm. Keeping your balance(s) at 10% or less of your credit limit is ideal in the eyes of FICO. If you close an account with good history and a good utilization it will no longer factor in to that 30% portion and could potentially damage your FICO score. Again, this is all without knowing the rest of your credit report. These things are super complicated and no one truly knows how much each action will affect a score being that the algorithm is highly confidential and proprietary to Fair Issac.
It also depends on your credit history with the green card. Is it in good standing or do you have balance, high utililzation, have you been late 30 days or more?
If that is the account with the longest credit history (3 years) then yes that will negatively affect you as well.

Credit does not equal debt until you start running up the balances and then your debt to income ratio will be affect and this will impact your score too.