A Muslim is a follower and believer of Islam. A non-Muslim from a Muslim country is not a Muslim, while a Muslim is. I really don't get why you're asking this. I assume the answer would be obvious if we were talking about Christianity or libertarianism or any other distinct set of beliefs: if you believe in them, you are a believer.
Whether any belief system has the potential to cause problems for a society depends on the content of that belief system and the content of the belief systems already present in that society. In the case of Islam in particular, there are two facts worth noting:
First, a strong case can be made for major compatibility problems of mainstream Sunni Islam (MAI) with Western societies. I won't go into details here, but very generally speaking, MAI has a theocratic component: Muslims should, in theory, strive towards the implementation of sharia law. As a body of laws and in terms of its axioms, sharia is simply incompatible with the Western legal tradition. I'm sure that what I'm saying here is not controversial among MAI Muslims. Your average Muslim (assuming he's honest) will corroborate this.
Second, Muslims come from a culture that is very unlike that of the US. Even if there were no incompatibilities, the bare fact that they're so culturally distant poses a barrier to the formation of the social bonds that are necessary for high trust, high cohesion communities and societies. In case that's not obvious: people generally bond more with others with whom they share the same cultural reference frame and state of mind.
Supposing you grant me these two arguments, then the potential for causing problems for a society is established. Whether that potential is actualized depends on the demographic weight a group has and the extent to which it is willing to compromise.
Speaking as a Dutchman (and realizing that the demographics of our Muslim population differ substantial from that of the US), it is clear to me that Muslims as a group cause problems in both senses. For example, in areas where there is a substantial Muslim demographic, there are now local political parties that explicitly cater to them. We never asked for this and we don't want it, but now we're stuck having to deal with it and with the social friction that comes with it. Another example: schools with substantial numbers of Muslim pupils are subject to great social pressures by this group, with some not being able to discuss certain topics anymore (Holocaust, criticism of Islam, cheering Muslim pupils during the aftermath of the Charlie Hebdo shootings) and concessions being made at the cost of native students such as defaulting to halal food. If you want sources for any of these claims, let me know.