The gravel road that leaves Blue Ridge Road at that location is marked "No Trespassing." The marking is on a sign in the center of a cable across the road, which is usually intended to make clear that the road itself, as well as the surrounding land, is private.
In Essex County, I use the Essex County GIS:
http://rpts-web.co.essex.ny.us/rpts/website/EssexCountyGISPublic/#
to determine ownership. Go to Planning and Zoning, Parcel and Property Use Data. It's quite useful. It also has very accurately drawn boundaries, which help clarify errors on many "hiking" maps.
The land immediately around the gravel road appears to be owned by a private owner. The larger parcel around and north of that is owned by the Nature Conservancy.
Two other approaches to Marsh Pond that you might consider are:
From Palmer Pond: Just west of Bessey Loop, there is state land access on Blue Ridge Road. (Some maps show a strip of private land north of the road, but that's incorrect. I have confirmed the state land access on the ground.) You could climb over the initial small hill, and then go north to pick up the Marsh Pond outlet. At least as far as the hilltop, the woods there are extremely open and scenic.
From Route 9: Refer to Adirondack Rock, the climbing guidebook, for the approach to North Hudson Dome from Route 9. This approach does involve a crossing of the Schroon River, which is usually about knee deep at the crossing; I do not know how that would be in winter. After crossing the river, you reach a walking culvert where the Marsh Pond outlet runs under the Northway. This is a nice, clean square box culvert. Once past the Northway, you could follow the outlet brook to the pond. I have followed this brook for about 1/2 mile, and again, the woods are magnificently open and scenic.
That's a beautiful area! Gotta love North Hudson for open woods; almost never a spruce thicket!
Have fun!