Why is the turtle doing these things?

There are two turtles in my Biology classroom in a tank, and one of them is sort of harassing the other. He (I’m guessing it’s a guy…) puts his “claws” or whatever they’re called out and he vibrates them in the other turtle’s face.

Also, I was watching him swim once, and he was just swimming around, and then this random black “sack” like thing appeared by his tail. It looked like it had little balls in it.

What’s up with him?