Hello I believe the substance you are talking about could possibly just be skin that your turtles have shed. I have two res and must change the tank about every other day when they are shedding. The actions you discribed sounded very much like the mating rituals. I would love to give them a new home for you if that is what it comes to. Never release them into the wild if they are pets. They will most likely die. They wouldn't know how or where to find food since it has always been given to them. let me know if I can help you in any way.