Is it Islamically permissible to say, "The owner of the mosque"? May Allah reward you on our behalf with all good.

It is permissible to use the phrase "owner of the mosque" to refer to someone who regularly attends it and maintains prayers within it, or to the donor who still retains ownership of the land.

It is not permissible to use this phrase to refer to the owner of a mosque who disposes of it as if it were his personal property, even if he was the one who built it. This is because a mosque becomes an endowment for Allah as soon as it is built and permission is given for prayer within it. The ownership of the endower is then removed from it, and he has no right to dispose of it except in ways that benefit the mosque.