If it's just the skeleton of the coral and there are no bits of flesh still left on it, you can definitely use it in your FW
tank. My mom used to have a few bits of coral skeleton in our FW
tank when i was younger.
However, i would advise bleaching it. That's what she did as well, and it would be best for you to do so as well. Not really worried about decaying from coral flesh, but what contaminants, if any, could still be on it, that would change your water chemistry, or worse, kill your tankmates.