Probably not a few decades. Silver will grow a case around it fast. That silver looks about 2 or 3 years in salt water. Just a guess of course but I have found silver far worse from beaches that are not decades old. If it was there decades it would be encased really thick with a dark coating, not a grey coat and probably mostly dissolved. You can tap it gently with a small hammer to remove the coat. It will pop off easy. Decades old silver will have dissolved with small rings like those. Very cool find though. I would leave them alone for show and tell.