Welcome
@rioila to the Sailboat Forum.
Congrats on your new to you boat.
Did you have the boat surveyed? Did the item come up on a list of concerns about the boat?
Yes there is surface rust on that nut. It is not a stainless part. The rod is attached to a steel bracket beneath the fiberglass you see around the nut. The bracket would be attached to the hull as the rigging is designed to transfer the forces of the mast and sail through the deck to the hull.
You might get one of those remote cameras (Harbor Freight sells them - nice ones for about $150) and look beneath the fiberglass to assess the level of rust/corrosion that is happening. Do you have a water leak in the area?
Remember you bought a boat that is 30 plus years old. The original hardware is going to have some wear on it. The photo is just a piece of the system. You will want to do a lot more looking/inspecting before you get excited about the issue. It is an issue that I would put high on my priority list to inspect and get a complete picture before I went into tearing stuff out.
If this is an area you are uncomfortable with you might want to find a rigger in the area to take a look. They will have seen this before and can give you a reasonable appraisal as to how serious the issue is.