Actually in central Russian regions most Caucasians are from independent south Caucasian republics and they mostly make tense rather than from Russian Muslim states.
What about people from Central Asia; do you think there is a problem with them too, or just Caucasians?
Well, compare Arabs and Indians in Europe, maybe the last ones make less problems but still there are many cultural, behavioral, hygienic differences which make things not easy. Maybe labor migrants entrance is not bad but there should be control of it, they should go for particular works in demand but they flood in on themselves and if they don't find a job they often to go to thieve (many come in with that purpose).