I believe the concept of giving the Slavs a local organized religion has been discussed recently. Of course, that thread dealt more with Not!Neoplatonism, but I think the arguments stated there apply here anyway.
Slavs were disorganized and on a considerably lower footing than the Arabs, their paganism, while notoriously inclusive, had few contacts with foreign faiths in the 7th century, and it was, much like the Baltic and Finnic equivalents, less of an actual religion and more of a loose collection of beliefs and concepts of worship, thus they had basically no experience with organized religion. They had no writing system, either, so you can forget the creation of any religious texts (which, while not mandatory for an organized religion, are extremely useful in preserving and spreading your faith, which is why most of current OTL religions have them).
The Slavs also, unlike the Arabs, did not have pre-existing states or proto-states (the first Slavic countries arose well after their migration slowed down) which could serve as a point of origin or base of operations for any Slavic "Islam".
And even if an organized stable (presumably) monotheistic religion arose in some Slavic tribe in the 7th century, a Slavic "Caliphate" would be much, much, much weaker and less of a threat to Byzantium and anyone else around (Avars?) than the Arabs were. The Arabian Peninsula had rich cities, trade routes to the rest of the world and technology not far from what was best at the time, all three of which the Slavic homeland, wherever it was, sorely lacked. The only thing really distinguishing it from all the other semi-nomadic peoples running around Central and Eastern Europe at the time, at least in the eyes of the Byzantines, was that they would have a funny faith that sounds similar to Christianity.