Well, the key question is: Do you believe that Muslims worship the same God that we do? I thought to be a heretic, one had to, at least on the surface, be of the same general faith. Are Muslims errant Christians or are they something completely different? Or, in the alternative, is a heretic simply anyone who disagrees with Catholics or Orthodox?
BTW, and please forgive the weird detour, but is Ishmael the "Abraham" of Muslims, or is Mohammed considered the Father?
In order to be a heretic, one has to be of the Apostolic faith to begin with and then develop an errant or un-orthodox teaching, such error being addressed by an ecumenical council and the heretic given a chance to recant his error and refuse to do so.
An Ecumenical Council is a college of Bishops of the entire Church, presided by the Pope (since the Second Council), and its decision is binding on the whole Church. A local Synod can excommunicate a heretical bishop but cannot call him a heretic but only schismatic.
To the best of my knowledge, Islam was never considered to be a Christian faith, not was it ever addressed as heresy by any of the seven E. Councils. Islam, like Judaism, is a different faith. They believe in one and the same God, the only God, but their revelation is considered defficient or false.