Can the other parent use an open children's aid society file against me in our parenting case?

An open file, by itself, isn't proof of anything - it means a concern was reported and is being looked into, not that anything has been substantiated. That said, the other parent can still raise the existence of the investigation and the underlying concerns as part of their evidence in a parenting motion or trial, and ask the court to weigh it under the best-interests-of-the-child test, which expressly considers each parent's ability to care for and protect the child, including any family violence concerns.

Courts generally treat an unsubstantiated or still-open investigation cautiously, since being investigated is not the same as being found to have harmed a child - but that doesn't mean the court will ignore it. The family court can also order that relevant CAS records be produced if a party asks. The safest response is not to dismiss it: cooperate with the society, keep your own records of what's happening, and get legal advice quickly, since refusing to engage or appearing evasive can itself be used to argue you aren't prioritizing the child's safety, regardless of how the investigation itself turns out.