Whatever is on your mind can be discussed.
However, remember mediations are hard work. This is an opportunity for you to solve your dispute. The mediator will help you and the other side to set an agenda at the beginning of the mediation. This will provide a framework for what you will discuss.
When the agenda is being set it is important that you speak up and make suggestions about issues you want to cover. Even if the other side does not think that they are relevant, if you think that they should be discussed, speak up.
It may be that during the mediation, what seemed to be a dispute about money, becomes a dispute about trust or communication or pride or safety. The mediator is trained to help discuss issues, which can be challenging. It may be necessary to talk about issues that underlie the dispute even if they are not in the open.