The doctrine of Immaculate Conception is not per se a heresy, in the sense that an Orthodox truly believes the Mother of God is truly "spotless" or "immaculate" as she never actually sinned. The true heresy in the formal definition of "Immaculate Conception" is that it's grounded on the concept of original sin, which is foreign to Orthodox doctrine, and even incompatible with the witness of the Church Fathers. Thus an Orthodox Christian can believe that the Theotokos actually never sinned, but NOT that she had no stain of original sin (which doesn't actually exist).
In case you don't understand the difference:
Original Sin means that every human inherits not only death (both spiritual and phisical) but also the guilt of Adam and Eve (thus even infants are guilty and sinners). For this the RC have so long struggled with this doctrine establishing that Mary didn't have this original sin since conception to preserve her holiness, or Pope Benedict XVI was obliged to justify the denial of Limbus (a place where non-baptised children were thought to go after death) saying that God must somehow intervene in their favour to rescue them...
Ancestral Sin is the name often given to our counterpart of the Original Sin doctrine, which is far less complicated in essence, since humans don't inherit Adam's guilt until they voluntarily sin for the first time. Sin is more a personal way to break the spiritual communion with God, and we can't be guilty of what others did. Nonetheless sin is a part of our world corrupted by the sin of our progenitors, and thus all are equally tempted. The only intervention of God in Mary's life was, according to some apocryphal traditions of the Church ("the Protoevangelium of saint James", although folkloristic and fantastic in nature, contains a good portrayal of this idea) that she was somehow sanctified and protected since her conception up to the Annunciation (for example, receiving only angelical food aand being dedicated by her blessed parents to the Lord as a temple virgin). And God provides no extra grace for non-baptised infants: since they never ratified the guilt of sin - having in truth no will to reject God but living somehow like angels with a natural predisposition to God - they automatically die in the Lord.
I hope this could clear up your mind (I beg your pardon if my rusty English makes it difficult to understand)
In Christ, Alex