If your ex-partner has contacted a mediator about discussing issues following separation, then you be asked by that mediator to attend a meeting, sometimes known as a Mediation Information and Assessment Meeting (MIAM). This is a standard part of the mediation process. It means that your ex-partner either will attend, or has attended, a MIAM. You can find out more about these meetings here.
If the mediator is registered with the FMC, then contact the mediator to discuss the invitation or arrange an appointment.
If you or your ex-partner are not eligible for legal aid, you will have to pay for the meeting. Although this can come as an unwelcome surprise, attending a MIAM can save you money in the long run, because the information the mediator can provide in this first meeting can help you resolve any issues in the most cost-effective way that is right for you and your family.
You should usually go to the mediator that your ex-partner has seen but, if you have concerns that they are not right (for example, because they do not offer legal aid when you are eligible to claim it), or because you know the mediator and there is therefore a conflict of interest, then you can see a different mediator. It is sensible to speak to your ex-partner, as well as the mediator who your ex-partner has contacted, about the reasons for wanting to see a different mediator. Make sure the mediator you see is FMC Registered.
If the mediator who has contacted you is not registered with the FMC, then you should contact an FMC Registered Mediator (see the search facility here) about seeing them instead. Again, it is sensible to let your ex-partner know why you want to see a different mediator.