The ruling on praying in a masjid with a grave in it is as follows:
If the masjid was built on the grave, this masjid must be demolished by the authorities and it is forbidden to pray in it and the prayer is not accepted.
But if the masjid was there first and then they introduced the grave, the grave must be dug out. It is permissible to pray in such a masjid providing that the grave is not in the direction of the Qibla and it is not at the front of the masjid.