There's an argument that "you're not a Muslim" is a statement of condemnation, not of fact. Not "there is some secret definition of Muslim I have and you don't fit it" but rather "I don't consider you to be Ingroup any more, you are Outgroup now". Basically a metaphorical exile.
Could be wrong, though. I'm not in their head.
Either way, it's not a fallacy, any more than calling someone a motherfucker because they cut you off in traffic is an ad hominem. Fallacy requires an argument being made.