Since the cache is located outside the Greenway's gates, it should be accessible at any time. The gates typically close at dark. There is a parking lot close by, but if the gates are closed, you can just park at the gates.
Although the creek is named 6-Mile, the Greenway is only about a mile one way. The Greenway frequently floods after big rains, but that shouldn't affect the cache, which is not on the Greenway.
In the summer, watch out for poison ivy.