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"Accident" here is used in the predicamental sense, rather than as one of Aristotle's categories. An "accident" is something predicated to a certain being which is not contained in its definition/quiddity/essence.
As we have already established that there is no composition of genus/species within God, we now ask if there are any accidents that flow from the essence or are superadded to it.
The difficulty present here is stated in De Potentia, "Whether good, wise, just, and the like predicate an accident in God?" For, many of the things we predicate to God are accidents among creatures.