What does it mean that it is closed on Mondays? Does that mean buildings or that you can't go walking at all?

The trails are open daily from 7 am to dusk (just after sunset).

And all trails are accessible through the campground even if building is closed.

Trails should be open.

You probably won't be able to access the trails that go around the pond and out to the bay. They start from inside the building. However, the longer trail that leaves to right of the building "should" be accessible.

They exhibit hall is closed, trails should still be open.