It's not Slavc in the sense that it was not created by Slavs. But it also is a Slavic religion because it is practiced by millions of Slavs. But I'd still be hesitant to call it 'Slavic'.

Millions of Slavs are muzlim because Islam has been forced onto the Slavs by centuries of foreign imperialism(Ottoman Empire). Izlam is a foreign import to Slavic culture