Well, there is "only one Islam". Can we use that Islam to stereotype Muslims? Or do we have to assume that (to paraphrase) "interpretations of Islam are like navels, everybody has one"?

I would lean towards the latter, which means you can't really have any prejudices against Muslims. Heck, I don't think you can even assume every Muslims believes that "there is no God except Allah and Muhammed is the messenger of Allah", since there are so many "secular Muslims" where I live. So whenever I disagree with certain interpretations of Islam I try to limit my criticism to Islam as a doctrine, rather than Muslims (as in the people). So I might well be an Islamophobe, but I would be severely insulted if someone called me a Muslimophobe .