What is the ruling on praying in a mosque built upon a very old cemetery whose traces have vanished and in which there are no leveled graves, and what is the ruling on building houses there, and is it obligatory to boycott this mosque?

If any bones of the deceased remain, the land is consecrated for them, and it is not permissible to exhume it, build upon it, or walk over it. All structures built on the graveyard, whether dwellings or mosques, must be removed. The action of the youth in abandoning prayer in this mosque is correct.