Why is the crescent moon used on the top of almost every masjid as if it is an Islamic logo? Is this practice Islamic and what is its origin? assalam

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Muslim architects have been using the sign of crescent in their art only to decorate buildings. When Muslim rulers conquered Christian areas, they observed the sign of the Cross on churches. Thus, they might be pleased with the sign of crescent and from this you may know that using the crescent is not a religious matter and there is no authenticity for it in Islam. But this may signify the Mosques in non-Muslim countries. An outsider may easily identify it. You should know that the picture of the crescent or any other picture does not have any relation with faith and worship. If anybody sees it as a religious matter, then, this is considered an innovation in religion, which is a big sin.

Allah knows best.